A list is a sequence of or more elements, expressed in this form: [a 1 , a 2 , a 3 , ... , a n ], where each a i
is or more consecutive digits or lowercase letters. i.e. each list begins with a left square bracket,
then zero or more elements separated by a single comma, followed by a right square bracket. There will
be no whitespace characters (spaces, TABs etc) within a list.
In this problem, we use two list operations: append (++) and remove (--).
1. A ++ B: append elements in B to the end of A.
2. A -- B: remove all the elements in B, from A. If something appears more than once in B, remove it that
many times in A. If there are many equal elements in A to choose from, remove them from left to right
(until all occurrences are removed, or there is no need to remove more elements).
Your task is to write a calculator, evaluating simple expressions or the form "list1 ++ list2" or "list1 --
list2".