Change "instances" class variables of Smybols and children of Symbols to Dict instead of List. Start expand.py (not completed)

python_symb/TreeModification/expand.py

@@ -0,0 +1,14 @@
+from python_symb.Expressions.expr import Expr
+from python_symb.MathTypes.symbols import Var
+from python_symb.MathTypes.operator_file import Operator, BinOperator, Add, Mul
+
+def expand(expr: Expr) -> Expr:
+    """
+    Expand an expression
+    :param expr: expression to expand
+    :return: expanded expression
+    """
+
+    if expr.is_leaf:
+        return expr
+