Do not inadvertently simplify expressions when computing free symbols

dace/sdfg/sdfg.py

@@ -273,7 +273,7 @@ def used_symbols(self, all_symbols: bool) -> Set[str]:
         rhs_symbols = set()
         for lhs, rhs in self.assignments.items():
             # Always add LHS symbols to the set of candidate free symbols
-            rhs_symbols |= symbolic.free_symbols_and_functions(rhs)
+            rhs_symbols |= set(map(str, dace.symbolic.symbols_in_ast(ast.parse(rhs))))
             # Add the RHS to the set of candidate defined symbols ONLY if it has not been read yet
             # This also solves the ordering issue that may arise in cases like the 3rd example above
             if lhs not in cond_symbols and lhs not in rhs_symbols: