[poincare] Expression::ExpressionWithoutSymbol does not take into

account SystemSymbol (UnknownX)

poincare/include/poincare/symbol.h

@@ -70,6 +70,7 @@ public:
     return Expression();
   }
   // Symbol properties
+  bool isSystemSymbol() const { return name()[0] == SpecialSymbols::UnknownX && name()[1] == 0; }
   static bool isSeriesSymbol(const char * c);
   static bool isRegressionSymbol(const char * c);
   bool matches(ExpressionTest test, Context & context) const;

poincare/src/expression.cpp

@@ -226,6 +226,7 @@ void Expression::defaultSetChildrenInPlace(Expression other) {
 bool Expression::hasReplaceableSymbols(Context & context) const {
   return recursivelyMatches([](const Expression e, Context & context, bool replaceSymbols) {
       return (e.type() == ExpressionNode::Type::Symbol
+          && !static_cast<const Symbol &>(e).isSystemSymbol()
           && !context.expressionForSymbol(static_cast<const Symbol &>(e)).isUninitialized())
       || (e.type() == ExpressionNode::Type::Function
            && !context.expressionForSymbol(static_cast<const Function &>(e)).isUninitialized());

poincare/src/symbol.cpp

@@ -204,6 +204,9 @@ int Symbol::getPolynomialCoefficients(Context & context, const char * symbolName
 }
 
 Expression Symbol::replaceReplaceableSymbols(Context & context) {
+  if (isSystemSymbol()) {
+    return *this;
+  }
   Expression e = context.expressionForSymbol(*this);
   if (e.isUninitialized()) {
     return *this;