add __getitem__ and __setitem__ to Expression

pymath/expression.py

@@ -147,6 +147,15 @@ class Expression(object):
                 return 0
         return  1
 
+    # -----------
+    # Expression act as container from self.postfix_tokens
+
+    def __getitem__(self, index):
+        return self.postfix_tokens[index]
+
+    def __setitem__(self, index, value):
+        self.postfix_tokens[index] = value
+
     # -----------
     # Some math manipulations