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

python_symb/Expressions/tree.py

@@ -24,6 +24,10 @@ class Tree(ABC):
         self.depth_first_order = depth_first_order
         self.children = children if children else []
 
+    @property
+    def is_leaf(self) -> bool:
+        return not self.children
+
     def __repr__(self) -> str:
         return f'Tree({self.value}, {self.children})' if self.children else f'Tree({self.value})'