Added a transformation, which should get rid of sympy expressions.

Although expressions have args and function as a tree, args cannot be
modified. So tree-transformations are not possible.
Sympy has subs function, however it replaces all matched in any
subexpression, of the expression (i.e. not only in args but in args args
and so on).

ast.py

@@ -302,7 +302,7 @@ class SympyAssignment(Node):
     def lhs(self, newValue):
         self._lhsSymbol = newValue
         self._isDeclaration = True
-        if isinstance(self._lhsSymbol, Field.Access) or isinstance(self._lhsSymbol, Indexed):
+        if isinstance(self._lhsSymbol, Field.Access) or isinstance(self._lhsSymbol, sp.Indexed):
             self._isDeclaration = False
 
     @property