diff --git a/apps/calculation/edit_expression_controller.h b/apps/calculation/edit_expression_controller.h
index f9276cdcc..aa4135288 100644
--- a/apps/calculation/edit_expression_controller.h
+++ b/apps/calculation/edit_expression_controller.h
@@ -2,10 +2,10 @@
 #define CALCULATION_EDIT_EXPRESSION_CONTROLLER_H
 
 #include <escher.h>
+#include "../shared/text_field_delegate.h"
 #include "history_controller.h"
 #include "calculation_store.h"
 #include "text_field.h"
-#include "../shared/text_field_delegate.h"
 
 namespace Calculation {
 class HistoryController;
diff --git a/apps/graph/app.cpp b/apps/graph/app.cpp
index 4284473bd..3906c9822 100644
--- a/apps/graph/app.cpp
+++ b/apps/graph/app.cpp
@@ -33,7 +33,10 @@ InputViewController * App::inputViewController() {
 }
 
 Context * App::localContext() {
-  return &m_xContext;
+  if (m_tabViewController.activeTab() == 0) {
+    return &m_xContext;
+  }
+  return TextFieldDelegateApp::localContext();
 }
 
 }
diff --git a/apps/i18n.cpp b/apps/i18n.cpp
index fb2465023..25536c107 100644
--- a/apps/i18n.cpp
+++ b/apps/i18n.cpp
@@ -22,7 +22,7 @@ const char * messages[196][3] {
   {"Valider", "Confirm", "Confirmar"},
   {"Suivant", "Next", "Siguiente"},
   {"Attention a la syntaxe", "Syntax error", "Error sintactico"},
-  {"Error mathematique", "Math error", "Error matematico"},
+  {"Erreur mathematique", "Math error", "Error matematico"},
 
   /* Variables */
   {"Variables", "Variables", "Variables"},
diff --git a/apps/probability/parameters_controller.cpp b/apps/probability/parameters_controller.cpp
index 089bf0cb9..438f63988 100644
--- a/apps/probability/parameters_controller.cpp
+++ b/apps/probability/parameters_controller.cpp
@@ -151,9 +151,11 @@ bool ParametersController::setParameterAtIndex(int parameterIndex, float f) {
 }
 
 bool ParametersController::textFieldDidFinishEditing(TextField * textField, const char * text) {
-  FloatParameterController::textFieldDidFinishEditing(textField, text);
-  m_selectableTableView.reloadData();
-  return true;
+  if (FloatParameterController::textFieldDidFinishEditing(textField, text)) {
+    m_selectableTableView.reloadData();
+    return true;
+  }
+  return false;
 }
 
 void ParametersController::buttonAction() {
diff --git a/apps/regression/go_to_parameter_controller.cpp b/apps/regression/go_to_parameter_controller.cpp
index 0e851622f..f0d45e799 100644
--- a/apps/regression/go_to_parameter_controller.cpp
+++ b/apps/regression/go_to_parameter_controller.cpp
@@ -111,7 +111,6 @@ bool GoToParameterController::textFieldDidFinishEditing(TextField * textField, c
 
 void GoToParameterController::viewWillAppear() {
   m_previousParameter = parameterAtIndex(0);
-  m_selectableTableView.reloadData();
 }
 
 void GoToParameterController::buttonAction() {
diff --git a/apps/sequence/app.cpp b/apps/sequence/app.cpp
index 8fea0a214..9ca86b5a2 100644
--- a/apps/sequence/app.cpp
+++ b/apps/sequence/app.cpp
@@ -30,8 +30,11 @@ InputViewController * App::inputViewController() {
   return &m_inputViewController;
 }
 
-LocalContext * App::localContext() {
-  return &m_nContext;
+Context * App::localContext() {
+  if (m_tabViewController.activeTab() == 0) {
+    return &m_nContext;
+  }
+  return TextFieldDelegateApp::localContext();
 }
 
 const char * App::XNT() {
diff --git a/apps/sequence/app.h b/apps/sequence/app.h
index 3da374b10..ce266f7c4 100644
--- a/apps/sequence/app.h
+++ b/apps/sequence/app.h
@@ -16,7 +16,7 @@ class App : public Shared::TextFieldDelegateApp {
 public:
   App(Container * container, Poincare::Context * context);
   InputViewController * inputViewController();
-  LocalContext * localContext() override;
+  Poincare::Context * localContext() override;
   const char * XNT() override;
 private:
   SequenceStore m_sequenceStore;
diff --git a/apps/shared/float_parameter_controller.cpp b/apps/shared/float_parameter_controller.cpp
index e60ead322..299f63ce2 100644
--- a/apps/shared/float_parameter_controller.cpp
+++ b/apps/shared/float_parameter_controller.cpp
@@ -24,18 +24,17 @@ View * FloatParameterController::view() {
 }
 
 void FloatParameterController::didBecomeFirstResponder() {
+  m_selectableTableView.reloadData();
   m_selectableTableView.selectCellAtLocation(0, 0);
   app()->setFirstResponder(&m_selectableTableView);
 }
 
-void FloatParameterController::viewWillAppear() {
-  m_selectableTableView.reloadData();
-}
-
 bool FloatParameterController::handleEvent(Ion::Events::Event event) {
   if (event == Ion::Events::Back) {
     for (int i = 0; i < numberOfRows()-1; i++) {
-      setParameterAtIndex(i, previousParameterAtIndex(i));
+      if (parameterAtIndex(i) != previousParameterAtIndex(i)) {
+        setParameterAtIndex(i, previousParameterAtIndex(i));
+      }
     }
     stackController()->pop();
     return true;
diff --git a/apps/shared/float_parameter_controller.h b/apps/shared/float_parameter_controller.h
index 5d2bf5955..25f2e4139 100644
--- a/apps/shared/float_parameter_controller.h
+++ b/apps/shared/float_parameter_controller.h
@@ -14,7 +14,6 @@ class FloatParameterController : public ViewController, public ListViewDataSourc
 public:
   FloatParameterController(Responder * parentResponder, I18n::Message okButtonText = I18n::Message::Ok);
   View * view() override;
-  void viewWillAppear() override;
   void didBecomeFirstResponder() override;
   bool handleEvent(Ion::Events::Event event) override;
 
diff --git a/apps/shared/range_parameter_controller.cpp b/apps/shared/range_parameter_controller.cpp
index 488e944e6..5222bdcec 100644
--- a/apps/shared/range_parameter_controller.cpp
+++ b/apps/shared/range_parameter_controller.cpp
@@ -62,9 +62,11 @@ void RangeParameterController::willDisplayCellForIndex(HighlightCell * cell, int
 }
 
 bool RangeParameterController::textFieldDidFinishEditing(TextField * textField, const char * text) {
-  FloatParameterController::textFieldDidFinishEditing(textField, text);
-  m_selectableTableView.reloadData();
-  return true;
+  if (FloatParameterController::textFieldDidFinishEditing(textField, text)) {
+    m_selectableTableView.reloadData();
+    return true;
+  }
+  return false;
 }
 
 void RangeParameterController::tableViewDidChangeSelection(SelectableTableView * t, int previousSelectedCellX, int previousSelectedCellY) {
diff --git a/apps/shared/text_field_delegate_app.cpp b/apps/shared/text_field_delegate_app.cpp
index aa9f1526d..b593bc837 100644
--- a/apps/shared/text_field_delegate_app.cpp
+++ b/apps/shared/text_field_delegate_app.cpp
@@ -29,16 +29,19 @@ bool TextFieldDelegateApp::cursorInToken(TextField * textField, const char * tok
   const char * text = textField->text();
   int location = textField->cursorLocation();
   int tokenLength = strlen(token);
-  while (text[location] != '(') {
-    location --;
-  }
-  if (location - tokenLength < 0) {
-    return false;
-  }
-  char previousToken[10];
-  strlcpy(previousToken, text+location-tokenLength, tokenLength+1);
-  if (strcmp(previousToken, token) == 0) {
-    return true;
+  while (location >= 0) {
+    while (text[location] != '(') {
+      location --;
+    }
+    if (location - tokenLength < 0) {
+      return false;
+    }
+    char previousToken[10];
+    strlcpy(previousToken, text+location-tokenLength, tokenLength+1);
+    if (strcmp(previousToken, token) == 0) {
+      return true;
+    }
+    location--;
   }
   return false;
 }
@@ -46,7 +49,7 @@ bool TextFieldDelegateApp::cursorInToken(TextField * textField, const char * tok
 bool TextFieldDelegateApp::textFieldDidReceiveEvent(TextField * textField, Ion::Events::Event event) {
   if ((event == Ion::Events::OK || event == Ion::Events::EXE) && textField->isEditing()) {
     Expression * exp = Expression::parse(textField->text());
-    if (exp == nullptr) {
+    if (exp == nullptr || !exp->hasValidNumberOfArguments()) {
       if (textField->textLength() == 0) {
         return true;
       }
diff --git a/escher/include/escher/tab_view_controller.h b/escher/include/escher/tab_view_controller.h
index 7b464b61b..7a3aa0934 100644
--- a/escher/include/escher/tab_view_controller.h
+++ b/escher/include/escher/tab_view_controller.h
@@ -8,7 +8,7 @@ class TabViewController : public ViewController {
 public:
   TabViewController(Responder * parentResponder, ViewController * one, ViewController * two, ViewController * three, ViewController * four = nullptr);
   View * view() override;
-
+  int activeTab() const;
   void setSelectedTab(int8_t index);
   void setActiveTab(int8_t index, bool forceReactive = false);
   uint8_t numberOfTabs();
diff --git a/escher/src/editable_text_cell.cpp b/escher/src/editable_text_cell.cpp
index e80fd7947..76f2d7da3 100644
--- a/escher/src/editable_text_cell.cpp
+++ b/escher/src/editable_text_cell.cpp
@@ -39,10 +39,7 @@ View * EditableTextCell::subviewAtIndex(int index) {
 }
 
 void EditableTextCell::layoutSubviews() {
-  KDCoordinate width = bounds().width();
-  KDCoordinate height = bounds().height();
-  KDCoordinate textHeight = m_textField.minimalSizeForOptimalDisplay().height();
-  m_textField.setFrame(KDRect(k_separatorThickness, (height - textHeight)/2, width - k_separatorThickness, textHeight));
+  m_textField.setFrame(bounds());
 }
 
 void EditableTextCell::didBecomeFirstResponder() {
diff --git a/escher/src/tab_view_controller.cpp b/escher/src/tab_view_controller.cpp
index 88cc647fb..0d259e007 100644
--- a/escher/src/tab_view_controller.cpp
+++ b/escher/src/tab_view_controller.cpp
@@ -69,6 +69,10 @@ TabViewController::TabViewController(Responder * parentResponder, ViewController
   }
 }
 
+int TabViewController::activeTab() const {
+  return m_activeChildIndex;
+}
+
 bool TabViewController::handleEvent(Ion::Events::Event event) {
   if (event == Ion::Events::Back) {
     if (app()->firstResponder() != this) {
diff --git a/escher/src/text_field.cpp b/escher/src/text_field.cpp
index c720783f1..5ddabb1a0 100644
--- a/escher/src/text_field.cpp
+++ b/escher/src/text_field.cpp
@@ -21,11 +21,15 @@ TextField::ContentView::ContentView(char * textBuffer, char * draftTextBuffer, s
 }
 
 void TextField::ContentView::drawRect(KDContext * ctx, KDRect rect) const {
-  ctx->fillRect(rect, m_backgroundColor);
+  KDColor bckCol = m_backgroundColor;
+  if (m_isEditing) {
+    bckCol = KDColorWhite;
+  }
+  ctx->fillRect(rect, bckCol);
   KDSize textSize = KDText::stringSize(text(), m_fontSize);
   KDPoint origin(m_horizontalAlignment*(m_frame.width() - textSize.width()-m_cursorView.minimalSizeForOptimalDisplay().width()),
       m_verticalAlignment*(m_frame.height() - textSize.height()));
-  ctx->drawString(text(), origin, m_fontSize, m_textColor, m_backgroundColor);
+  ctx->drawString(text(), origin, m_fontSize, m_textColor, bckCol);
 }
 
 void TextField::ContentView::reload() {
@@ -277,6 +281,8 @@ bool TextField::handleEvent(Ion::Events::Event event) {
   }
   if (event == Ion::Events::OK || event == Ion::Events::EXE) {
     if (isEditing()) {
+      char bufferText[ContentView::k_maxBufferSize];
+      strlcpy(bufferText, m_contentView.textBuffer(), ContentView::k_maxBufferSize);
       strlcpy(m_contentView.textBuffer(), m_contentView.draftTextBuffer(), m_contentView.bufferSize());
       int cursorLoc = cursorLocation();
       setEditing(false);
@@ -284,11 +290,13 @@ bool TextField::handleEvent(Ion::Events::Event event) {
         reloadScroll();
         return true;
       }
-      char buffer[ContentView::k_maxBufferSize];
-      strlcpy(buffer, m_contentView.textBuffer(), ContentView::k_maxBufferSize);
-      setText("");
+      /* if the text was refused (textFieldDidFinishEditing returned false, we
+       * reset the textfield in the same state as before */
+      char bufferDraft[ContentView::k_maxBufferSize];
+      strlcpy(bufferDraft, m_contentView.textBuffer(), ContentView::k_maxBufferSize);
+      setText(bufferText);
       setEditing(true);
-      setText(buffer);
+      setText(bufferDraft);
       setCursorLocation(cursorLoc);
       return true;
     }
diff --git a/poincare/include/poincare/binary_operation.h b/poincare/include/poincare/binary_operation.h
index f0359a346..5e62cdbc1 100644
--- a/poincare/include/poincare/binary_operation.h
+++ b/poincare/include/poincare/binary_operation.h
@@ -11,6 +11,7 @@ class BinaryOperation : public Expression {
 public:
   BinaryOperation(Expression ** operands, bool cloneOperands = true);
   ~BinaryOperation();
+  bool hasValidNumberOfArguments() const override;
   const Expression * operand(int i) const override;
   int numberOfOperands() const override;
   Expression * clone() const override;
diff --git a/poincare/include/poincare/binomial_coefficient.h b/poincare/include/poincare/binomial_coefficient.h
index 85eb2d58f..0224438a9 100644
--- a/poincare/include/poincare/binomial_coefficient.h
+++ b/poincare/include/poincare/binomial_coefficient.h
@@ -8,7 +8,7 @@ namespace Poincare {
 class BinomialCoefficient : public Function {
 public:
   BinomialCoefficient();
-  bool isValidNumberOfArguments(int numberOfArguments) override;
+  bool hasValidNumberOfArguments() const override;
   Type type() const override;
   Expression * cloneWithDifferentOperands(Expression ** newOperands,
     int numberOfOperands, bool cloneOperands = true) const override;
diff --git a/poincare/include/poincare/confidence_interval.h b/poincare/include/poincare/confidence_interval.h
index 227379084..113620d77 100644
--- a/poincare/include/poincare/confidence_interval.h
+++ b/poincare/include/poincare/confidence_interval.h
@@ -8,7 +8,7 @@ namespace Poincare {
 class ConfidenceInterval : public Function {
 public:
   ConfidenceInterval();
-  bool isValidNumberOfArguments(int numberOfArguments) override;
+  bool hasValidNumberOfArguments() const override;
   Type type() const override;
   Expression * cloneWithDifferentOperands(Expression ** newOperands,
     int numberOfOperands, bool cloneOperands = true) const override;
diff --git a/poincare/include/poincare/derivative.h b/poincare/include/poincare/derivative.h
index 3e4925449..4b4ba1a76 100644
--- a/poincare/include/poincare/derivative.h
+++ b/poincare/include/poincare/derivative.h
@@ -9,7 +9,7 @@ namespace Poincare {
 class Derivative : public Function {
 public:
   Derivative();
-  bool isValidNumberOfArguments(int numberOfArguments) override;
+  bool hasValidNumberOfArguments() const override;
   Type type() const override;
   Expression * cloneWithDifferentOperands(Expression ** newOperands,
       int numberOfOperands, bool cloneOperands = true) const override;
diff --git a/poincare/include/poincare/division_quotient.h b/poincare/include/poincare/division_quotient.h
index facb81026..08a50f2a8 100644
--- a/poincare/include/poincare/division_quotient.h
+++ b/poincare/include/poincare/division_quotient.h
@@ -8,7 +8,7 @@ namespace Poincare {
 class DivisionQuotient : public Function {
 public:
   DivisionQuotient();
-  bool isValidNumberOfArguments(int numberOfArguments) override;
+  bool hasValidNumberOfArguments() const override;
   Type type() const override;
   Expression * cloneWithDifferentOperands(Expression ** newOperands,
     int numberOfOperands, bool cloneOperands = true) const override;
diff --git a/poincare/include/poincare/division_remainder.h b/poincare/include/poincare/division_remainder.h
index f8006f2d8..b44552df1 100644
--- a/poincare/include/poincare/division_remainder.h
+++ b/poincare/include/poincare/division_remainder.h
@@ -8,7 +8,7 @@ namespace Poincare {
 class DivisionRemainder : public Function {
 public:
   DivisionRemainder();
-  bool isValidNumberOfArguments(int numberOfArguments) override;
+  bool hasValidNumberOfArguments() const override;
   Type type() const override;
   Expression * cloneWithDifferentOperands(Expression ** newOperands,
     int numberOfOperands, bool cloneOperands = true) const override;
diff --git a/poincare/include/poincare/expression.h b/poincare/include/poincare/expression.h
index f89f70300..a97688441 100644
--- a/poincare/include/poincare/expression.h
+++ b/poincare/include/poincare/expression.h
@@ -84,7 +84,7 @@ public:
   };
   static Expression * parse(char const * string);
   virtual ~Expression();
-
+  virtual bool hasValidNumberOfArguments() const = 0;
   ExpressionLayout * createLayout(FloatDisplayMode floatDisplayMode = FloatDisplayMode::Default, ComplexFormat complexFormat = ComplexFormat::Default) const; // Returned object must be deleted
   virtual const Expression * operand(int i) const = 0;
   virtual int numberOfOperands() const = 0;
diff --git a/poincare/include/poincare/function.h b/poincare/include/poincare/function.h
index c102bab3c..0bd3b52ab 100644
--- a/poincare/include/poincare/function.h
+++ b/poincare/include/poincare/function.h
@@ -15,7 +15,7 @@ public:
   ~Function();
   void setArgument(Expression ** args, int numberOfArguments, bool clone = true);
   void setArgument(ListData * listData, bool clone = true);
-  virtual bool isValidNumberOfArguments(int numberOfArguments);
+  bool hasValidNumberOfArguments() const override;
   const Expression * operand(int i) const override;
   int numberOfOperands() const override;
   Expression * clone() const override;
diff --git a/poincare/include/poincare/great_common_divisor.h b/poincare/include/poincare/great_common_divisor.h
index 32e17b3e0..01af80945 100644
--- a/poincare/include/poincare/great_common_divisor.h
+++ b/poincare/include/poincare/great_common_divisor.h
@@ -8,7 +8,7 @@ namespace Poincare {
 class GreatCommonDivisor : public Function {
 public:
   GreatCommonDivisor();
-  bool isValidNumberOfArguments(int numberOfArguments) override;
+  bool hasValidNumberOfArguments() const override;
   Type type() const override;
   Expression * cloneWithDifferentOperands(Expression ** newOperands,
     int numberOfOperands, bool cloneOperands = true) const override;
diff --git a/poincare/include/poincare/integral.h b/poincare/include/poincare/integral.h
index 7a775339e..8d4f815f6 100644
--- a/poincare/include/poincare/integral.h
+++ b/poincare/include/poincare/integral.h
@@ -9,7 +9,7 @@ namespace Poincare {
 class Integral : public Function {
 public:
   Integral();
-  bool isValidNumberOfArguments(int numberOfArguments) override;
+  bool hasValidNumberOfArguments() const override;
   Type type() const override;
   Expression * cloneWithDifferentOperands(Expression ** newOperands,
       int numberOfOperands, bool cloneOperands = true) const override;
diff --git a/poincare/include/poincare/leaf_expression.h b/poincare/include/poincare/leaf_expression.h
index 4d1888175..a078d7832 100644
--- a/poincare/include/poincare/leaf_expression.h
+++ b/poincare/include/poincare/leaf_expression.h
@@ -7,6 +7,7 @@ namespace Poincare {
 
 class LeafExpression : public Expression {
 public:
+  bool hasValidNumberOfArguments() const override;
   const Expression * operand(int i) const override;
   int numberOfOperands() const override;
   Expression * cloneWithDifferentOperands(Expression** newOperands,
diff --git a/poincare/include/poincare/least_common_multiple.h b/poincare/include/poincare/least_common_multiple.h
index 5c4588065..66f6e63c2 100644
--- a/poincare/include/poincare/least_common_multiple.h
+++ b/poincare/include/poincare/least_common_multiple.h
@@ -8,7 +8,7 @@ namespace Poincare {
 class LeastCommonMultiple : public Function {
 public:
   LeastCommonMultiple();
-  bool isValidNumberOfArguments(int numberOfArguments) override;
+  bool hasValidNumberOfArguments() const override;
   Type type() const override;
   Expression * cloneWithDifferentOperands(Expression ** newOperands,
     int numberOfOperands, bool cloneOperands = true) const override;
diff --git a/poincare/include/poincare/logarithm.h b/poincare/include/poincare/logarithm.h
index c61c3db07..3c597f1aa 100644
--- a/poincare/include/poincare/logarithm.h
+++ b/poincare/include/poincare/logarithm.h
@@ -8,7 +8,7 @@ namespace Poincare {
 class Logarithm : public Function {
 public:
   Logarithm();
-  bool isValidNumberOfArguments(int numberOfArguments) override;
+  bool hasValidNumberOfArguments() const override;
   Type type() const override;
   Expression * cloneWithDifferentOperands(Expression ** newOperands,
       int numberOfOperands, bool cloneOperands = true) const override;
diff --git a/poincare/include/poincare/matrix.h b/poincare/include/poincare/matrix.h
index 99c26a84e..2fba2d6f8 100644
--- a/poincare/include/poincare/matrix.h
+++ b/poincare/include/poincare/matrix.h
@@ -12,6 +12,7 @@ public:
   Matrix(MatrixData * matrixData);
   Matrix(Expression ** newOperands, int numberOfOperands, int m_numberOfColumns, int m_numberOfRows, bool cloneOperands);
   ~Matrix();
+  bool hasValidNumberOfArguments() const override;
   const Expression * operand(int i) const override;
   int numberOfOperands() const override;
   Expression * clone() const override;
diff --git a/poincare/include/poincare/nth_root.h b/poincare/include/poincare/nth_root.h
index b3c5ea248..a7ab5e4cd 100644
--- a/poincare/include/poincare/nth_root.h
+++ b/poincare/include/poincare/nth_root.h
@@ -8,7 +8,7 @@ namespace Poincare {
 class NthRoot : public Function {
 public:
   NthRoot();
-  bool isValidNumberOfArguments(int numberOfArguments) override;
+  bool hasValidNumberOfArguments() const override;
   Type type() const override;
   Expression * cloneWithDifferentOperands(Expression ** newOperands,
     int numberOfOperands, bool cloneOperands = true) const override;
diff --git a/poincare/include/poincare/opposite.h b/poincare/include/poincare/opposite.h
index e55e52874..a102275e6 100644
--- a/poincare/include/poincare/opposite.h
+++ b/poincare/include/poincare/opposite.h
@@ -10,6 +10,7 @@ class Opposite : public Expression {
 public:
   Opposite(Expression * operand, bool cloneOperands = true);
   ~Opposite();
+  bool hasValidNumberOfArguments() const override;
   const Expression * operand(int i) const override;
   int numberOfOperands() const override;
   Expression * clone() const override;
diff --git a/poincare/include/poincare/parenthesis.h b/poincare/include/poincare/parenthesis.h
index de9945e34..d0e0b4aea 100644
--- a/poincare/include/poincare/parenthesis.h
+++ b/poincare/include/poincare/parenthesis.h
@@ -9,6 +9,7 @@ class Parenthesis : public Expression {
 public:
   Parenthesis(Expression * operand, bool cloneOperands = true);
   ~Parenthesis();
+  bool hasValidNumberOfArguments() const override;
   const Expression * operand(int i) const override;
   int numberOfOperands() const override;
   Expression * clone() const override;
diff --git a/poincare/include/poincare/permute_coefficient.h b/poincare/include/poincare/permute_coefficient.h
index f1358e135..fe3a8782c 100644
--- a/poincare/include/poincare/permute_coefficient.h
+++ b/poincare/include/poincare/permute_coefficient.h
@@ -8,7 +8,7 @@ namespace Poincare {
 class PermuteCoefficient : public Function {
 public:
   PermuteCoefficient();
-  bool isValidNumberOfArguments(int numberOfArguments) override;
+  bool hasValidNumberOfArguments() const override;
   Type type() const override;
   Expression * cloneWithDifferentOperands(Expression ** newOperands,
     int numberOfOperands, bool cloneOperands = true) const override;
diff --git a/poincare/include/poincare/prediction_interval.h b/poincare/include/poincare/prediction_interval.h
index 355b95b97..e00022850 100644
--- a/poincare/include/poincare/prediction_interval.h
+++ b/poincare/include/poincare/prediction_interval.h
@@ -8,7 +8,7 @@ namespace Poincare {
 class PredictionInterval : public Function {
 public:
   PredictionInterval();
-  bool isValidNumberOfArguments(int numberOfArguments) override;
+  bool hasValidNumberOfArguments() const override;
   Type type() const override;
   Expression * cloneWithDifferentOperands(Expression ** newOperands,
     int numberOfOperands, bool cloneOperands = true) const override;
diff --git a/poincare/include/poincare/product.h b/poincare/include/poincare/product.h
index f00cf23df..13e38b898 100644
--- a/poincare/include/poincare/product.h
+++ b/poincare/include/poincare/product.h
@@ -8,7 +8,7 @@ namespace Poincare {
 class Product : public Function {
 public:
   Product();
-  bool isValidNumberOfArguments(int numberOfArguments) override;
+  bool hasValidNumberOfArguments() const override;
   Type type() const override;
   Expression * cloneWithDifferentOperands(Expression ** newOperands,
       int numberOfOperands, bool cloneOperands = true) const override;
diff --git a/poincare/include/poincare/round.h b/poincare/include/poincare/round.h
index 884262988..5150726ae 100644
--- a/poincare/include/poincare/round.h
+++ b/poincare/include/poincare/round.h
@@ -8,7 +8,7 @@ namespace Poincare {
 class Round : public Function {
 public:
   Round();
-  bool isValidNumberOfArguments(int numberOfArguments) override;
+  bool hasValidNumberOfArguments() const override;
   Type type() const override;
   Expression * cloneWithDifferentOperands(Expression ** newOperands,
     int numberOfOperands, bool cloneOperands = true) const override;
diff --git a/poincare/include/poincare/store.h b/poincare/include/poincare/store.h
index e8928ca54..824b46527 100644
--- a/poincare/include/poincare/store.h
+++ b/poincare/include/poincare/store.h
@@ -10,6 +10,7 @@ class Store : public Expression {
 public:
   Store(Symbol * symbol, Expression * value, bool clone = true);
   ~Store();
+  bool hasValidNumberOfArguments() const override;
   Type type() const override;
   const Expression * operand(int i) const override;
   int numberOfOperands() const override;
diff --git a/poincare/include/poincare/sum.h b/poincare/include/poincare/sum.h
index c75fe0c07..402498116 100644
--- a/poincare/include/poincare/sum.h
+++ b/poincare/include/poincare/sum.h
@@ -8,7 +8,7 @@ namespace Poincare {
 class Sum : public Function {
 public:
   Sum();
-  bool isValidNumberOfArguments(int numberOfArguments) override;
+  bool hasValidNumberOfArguments() const override;
   Type type() const override;
   Expression * cloneWithDifferentOperands(Expression ** newOperands,
       int numberOfOperands, bool cloneOperands = true) const override;
diff --git a/poincare/src/binary_operation.cpp b/poincare/src/binary_operation.cpp
index 8c1f50652..be8c965d9 100644
--- a/poincare/src/binary_operation.cpp
+++ b/poincare/src/binary_operation.cpp
@@ -25,6 +25,10 @@ BinaryOperation::~BinaryOperation() {
   delete m_operands[0];
 }
 
+bool BinaryOperation::hasValidNumberOfArguments() const {
+  return m_operands[0]->hasValidNumberOfArguments() && m_operands[1]->hasValidNumberOfArguments();
+}
+
 int BinaryOperation::numberOfOperands() const {
   return 2;
 }
diff --git a/poincare/src/binomial_coefficient.cpp b/poincare/src/binomial_coefficient.cpp
index 56ee932d8..1af3292be 100644
--- a/poincare/src/binomial_coefficient.cpp
+++ b/poincare/src/binomial_coefficient.cpp
@@ -16,8 +16,8 @@ BinomialCoefficient::BinomialCoefficient() :
 {
 }
 
-bool BinomialCoefficient::isValidNumberOfArguments(int numberOfArguments) {
-  return (numberOfArguments == 2);
+bool BinomialCoefficient::hasValidNumberOfArguments() const {
+  return (m_numberOfArguments == 2);
 }
 
 Expression::Type BinomialCoefficient::type() const {
diff --git a/poincare/src/confidence_interval.cpp b/poincare/src/confidence_interval.cpp
index 185ed9185..411e58798 100644
--- a/poincare/src/confidence_interval.cpp
+++ b/poincare/src/confidence_interval.cpp
@@ -12,8 +12,8 @@ ConfidenceInterval::ConfidenceInterval() :
 {
 }
 
-bool ConfidenceInterval::isValidNumberOfArguments(int numberOfArguments) {
-  return (numberOfArguments == 2);
+bool ConfidenceInterval::hasValidNumberOfArguments() const {
+  return (m_numberOfArguments == 2);
 }
 
 Expression::Type ConfidenceInterval::type() const {
diff --git a/poincare/src/derivative.cpp b/poincare/src/derivative.cpp
index f7ac2127a..977a5a4e9 100644
--- a/poincare/src/derivative.cpp
+++ b/poincare/src/derivative.cpp
@@ -15,8 +15,8 @@ Derivative::Derivative() :
 {
 }
 
-bool Derivative::isValidNumberOfArguments(int numberOfArguments) {
-  return (numberOfArguments == 2);
+bool Derivative::hasValidNumberOfArguments() const {
+  return (m_numberOfArguments == 2);
 }
 
 Expression::Type Derivative::type() const {
diff --git a/poincare/src/division_quotient.cpp b/poincare/src/division_quotient.cpp
index 048e8fa5f..2d1c1023a 100644
--- a/poincare/src/division_quotient.cpp
+++ b/poincare/src/division_quotient.cpp
@@ -12,8 +12,8 @@ DivisionQuotient::DivisionQuotient() :
 {
 }
 
-bool DivisionQuotient::isValidNumberOfArguments(int numberOfArguments) {
-  return (numberOfArguments == 2);
+bool DivisionQuotient::hasValidNumberOfArguments() const {
+  return (m_numberOfArguments == 2);
 }
 
 Expression::Type DivisionQuotient::type() const {
diff --git a/poincare/src/division_remainder.cpp b/poincare/src/division_remainder.cpp
index 749644958..17defc038 100644
--- a/poincare/src/division_remainder.cpp
+++ b/poincare/src/division_remainder.cpp
@@ -12,8 +12,8 @@ DivisionRemainder::DivisionRemainder() :
 {
 }
 
-bool DivisionRemainder::isValidNumberOfArguments(int numberOfArguments) {
-  return (numberOfArguments == 2);
+bool DivisionRemainder::hasValidNumberOfArguments() const {
+  return (m_numberOfArguments == 2);
 }
 
 Expression::Type DivisionRemainder::type() const {
diff --git a/poincare/src/expression_parser.y b/poincare/src/expression_parser.y
index f18f4f93b..7d65240e0 100644
--- a/poincare/src/expression_parser.y
+++ b/poincare/src/expression_parser.y
@@ -141,7 +141,7 @@ exp:
   | MINUS exp        { $$ = new Poincare::Opposite($2, false); }
   | LEFT_PARENTHESIS exp RIGHT_PARENTHESIS     { $$ = new Poincare::Parenthesis($2, false); }
   | LEFT_BRACKET mtxData RIGHT_BRACKET { $$ = new Poincare::Matrix($2); }
-  | FUNCTION LEFT_PARENTHESIS lstData RIGHT_PARENTHESIS { if (!$1->isValidNumberOfArguments($3->numberOfOperands())) { $$ = new Poincare::Complex(Poincare::Complex::Float(NAN));} else {$$ = $1; $1->setArgument($3, true); delete $3;} }
+  | FUNCTION LEFT_PARENTHESIS lstData RIGHT_PARENTHESIS { $$ = $1; $1->setArgument($3, true); delete $3; }
 ;
 
 %%
diff --git a/poincare/src/function.cpp b/poincare/src/function.cpp
index 9c9d58e39..f2090836b 100644
--- a/poincare/src/function.cpp
+++ b/poincare/src/function.cpp
@@ -63,9 +63,9 @@ Function::~Function() {
   }
 }
 
-bool Function::isValidNumberOfArguments(int numberOfArguments) {
+bool Function::hasValidNumberOfArguments() const {
   /* By default, a function has one argument only */
-  return (numberOfArguments == 1);
+  return (m_numberOfArguments == 1);
 }
 
 Expression * Function::clone() const {
diff --git a/poincare/src/great_common_divisor.cpp b/poincare/src/great_common_divisor.cpp
index f694b7ac7..8ed2e7525 100644
--- a/poincare/src/great_common_divisor.cpp
+++ b/poincare/src/great_common_divisor.cpp
@@ -13,8 +13,8 @@ GreatCommonDivisor::GreatCommonDivisor() :
 {
 }
 
-bool GreatCommonDivisor::isValidNumberOfArguments(int numberOfArguments) {
-  return (numberOfArguments == 2);
+bool GreatCommonDivisor::hasValidNumberOfArguments() const {
+  return (m_numberOfArguments == 2);
 }
 
 Expression::Type GreatCommonDivisor::type() const {
diff --git a/poincare/src/integral.cpp b/poincare/src/integral.cpp
index 84531f810..7361017a2 100644
--- a/poincare/src/integral.cpp
+++ b/poincare/src/integral.cpp
@@ -19,8 +19,8 @@ Integral::Integral() :
 {
 }
 
-bool Integral::isValidNumberOfArguments(int numberOfArguments) {
-  return (numberOfArguments == 3);
+bool Integral::hasValidNumberOfArguments() const {
+  return (m_numberOfArguments == 3);
 }
 
 Expression::Type Integral::type() const {
diff --git a/poincare/src/leaf_expression.cpp b/poincare/src/leaf_expression.cpp
index 7b72f1a3a..e1a37d118 100644
--- a/poincare/src/leaf_expression.cpp
+++ b/poincare/src/leaf_expression.cpp
@@ -6,6 +6,10 @@ extern "C" {
 
 namespace Poincare {
 
+bool LeafExpression::hasValidNumberOfArguments() const {
+  return true;
+}
+
 int LeafExpression::numberOfOperands() const {
   return 0;
 }
diff --git a/poincare/src/least_common_multiple.cpp b/poincare/src/least_common_multiple.cpp
index 96ccaa75d..36213e235 100644
--- a/poincare/src/least_common_multiple.cpp
+++ b/poincare/src/least_common_multiple.cpp
@@ -13,8 +13,8 @@ LeastCommonMultiple::LeastCommonMultiple() :
 {
 }
 
-bool LeastCommonMultiple::isValidNumberOfArguments(int numberOfArguments) {
-  return (numberOfArguments == 2);
+bool LeastCommonMultiple::hasValidNumberOfArguments() const {
+  return (m_numberOfArguments == 2);
 }
 
 Expression::Type LeastCommonMultiple::type() const {
diff --git a/poincare/src/logarithm.cpp b/poincare/src/logarithm.cpp
index 0f265ffe7..db635ac91 100644
--- a/poincare/src/logarithm.cpp
+++ b/poincare/src/logarithm.cpp
@@ -16,8 +16,8 @@ Logarithm::Logarithm() :
 {
 }
 
-bool Logarithm::isValidNumberOfArguments(int numberOfArguments) {
-  return (numberOfArguments == 1 || numberOfArguments == 2);
+bool Logarithm::hasValidNumberOfArguments() const {
+  return (m_numberOfArguments == 1 || m_numberOfArguments == 2);
 }
 
 Expression::Type Logarithm::type() const {
diff --git a/poincare/src/matrix.cpp b/poincare/src/matrix.cpp
index 73c6bb10a..51fbb6317 100644
--- a/poincare/src/matrix.cpp
+++ b/poincare/src/matrix.cpp
@@ -21,6 +21,15 @@ Matrix::~Matrix() {
   delete m_matrixData;
 }
 
+bool Matrix::hasValidNumberOfArguments() const {
+  for (int i = 0; i < numberOfOperands(); i++) {
+    if (!operand(i)->hasValidNumberOfArguments()) {
+      return false;
+    }
+  }
+  return true;
+}
+
 Matrix::Matrix(Expression ** newOperands, int numberOfOperands, int numberOfColumns, int numberOfRows, bool cloneOperands)
 {
   m_matrixData = new MatrixData(newOperands, numberOfOperands, numberOfColumns, numberOfRows, cloneOperands);
diff --git a/poincare/src/nth_root.cpp b/poincare/src/nth_root.cpp
index 659dfd10a..b70ebaa66 100644
--- a/poincare/src/nth_root.cpp
+++ b/poincare/src/nth_root.cpp
@@ -16,8 +16,8 @@ NthRoot::NthRoot() :
 {
 }
 
-bool NthRoot::isValidNumberOfArguments(int numberOfArguments) {
-  return (numberOfArguments == 2);
+bool NthRoot::hasValidNumberOfArguments() const {
+  return (m_numberOfArguments == 2);
 }
 
 Expression::Type NthRoot::type() const {
diff --git a/poincare/src/opposite.cpp b/poincare/src/opposite.cpp
index 3cfc2977b..5be43f9d3 100644
--- a/poincare/src/opposite.cpp
+++ b/poincare/src/opposite.cpp
@@ -23,6 +23,10 @@ Opposite::~Opposite() {
   delete m_operand;
 }
 
+bool Opposite::hasValidNumberOfArguments() const {
+  return m_operand->hasValidNumberOfArguments();
+}
+
 const Expression * Opposite::operand(int i) const {
   assert(i == 0);
   return m_operand;
diff --git a/poincare/src/parenthesis.cpp b/poincare/src/parenthesis.cpp
index b6e1a2f51..b12aefe24 100644
--- a/poincare/src/parenthesis.cpp
+++ b/poincare/src/parenthesis.cpp
@@ -20,6 +20,10 @@ Parenthesis::~Parenthesis() {
   delete m_operand;
 }
 
+bool Parenthesis::hasValidNumberOfArguments() const {
+  return m_operand->hasValidNumberOfArguments();
+}
+
 int Parenthesis::numberOfOperands() const {
   return 1;
 }
diff --git a/poincare/src/permute_coefficient.cpp b/poincare/src/permute_coefficient.cpp
index 27cdea3bc..2fe6c2c16 100644
--- a/poincare/src/permute_coefficient.cpp
+++ b/poincare/src/permute_coefficient.cpp
@@ -13,8 +13,8 @@ PermuteCoefficient::PermuteCoefficient() :
 {
 }
 
-bool PermuteCoefficient::isValidNumberOfArguments(int numberOfArguments) {
-  return (numberOfArguments == 2);
+bool PermuteCoefficient::hasValidNumberOfArguments() const {
+  return (m_numberOfArguments == 2);
 }
 
 Expression::Type PermuteCoefficient::type() const {
diff --git a/poincare/src/prediction_interval.cpp b/poincare/src/prediction_interval.cpp
index fe93d73bb..c0b257080 100644
--- a/poincare/src/prediction_interval.cpp
+++ b/poincare/src/prediction_interval.cpp
@@ -12,8 +12,8 @@ PredictionInterval::PredictionInterval() :
 {
 }
 
-bool PredictionInterval::isValidNumberOfArguments(int numberOfArguments) {
-  return (numberOfArguments == 2);
+bool PredictionInterval::hasValidNumberOfArguments() const {
+  return (m_numberOfArguments == 2);
 }
 
 Expression::Type PredictionInterval::type() const {
diff --git a/poincare/src/product.cpp b/poincare/src/product.cpp
index b5878bde9..debb3fe9c 100644
--- a/poincare/src/product.cpp
+++ b/poincare/src/product.cpp
@@ -19,8 +19,8 @@ Product::Product() :
 {
 }
 
-bool Product::isValidNumberOfArguments(int numberOfArguments) {
-  return (numberOfArguments == 3);
+bool Product::hasValidNumberOfArguments() const {
+  return (m_numberOfArguments == 3);
 }
 
 Expression::Type Product::type() const {
diff --git a/poincare/src/round.cpp b/poincare/src/round.cpp
index d652c5a1d..e7cc61c64 100644
--- a/poincare/src/round.cpp
+++ b/poincare/src/round.cpp
@@ -12,8 +12,8 @@ Round::Round() :
 {
 }
 
-bool Round::isValidNumberOfArguments(int numberOfArguments) {
-  return (numberOfArguments == 2);
+bool Round::hasValidNumberOfArguments() const {
+  return (m_numberOfArguments == 2);
 }
 
 Expression::Type Round::type() const {
diff --git a/poincare/src/store.cpp b/poincare/src/store.cpp
index 4343e4b29..2050838cd 100644
--- a/poincare/src/store.cpp
+++ b/poincare/src/store.cpp
@@ -26,6 +26,10 @@ Store::~Store() {
   delete m_value;
 }
 
+bool Store::hasValidNumberOfArguments() const {
+  return m_value->hasValidNumberOfArguments();
+}
+
 Expression::Type Store::type() const {
   return Type::Store;
 }
diff --git a/poincare/src/sum.cpp b/poincare/src/sum.cpp
index 8c4c5f896..01053e574 100644
--- a/poincare/src/sum.cpp
+++ b/poincare/src/sum.cpp
@@ -19,8 +19,8 @@ Sum::Sum() :
 {
 }
 
-bool Sum::isValidNumberOfArguments(int numberOfArguments) {
-  return (numberOfArguments == 3);
+bool Sum::hasValidNumberOfArguments() const {
+  return (m_numberOfArguments == 3);
 }
 
 Expression::Type Sum::type() const {