new_contents: # Stack Abstract Data Type (ADT)
  
  # Stack() creates a new stack that is empty.
  #    It needs no parameters and returns an empty stack.
  # push(item) adds a new item to the top of the stack.
  #    It needs the item and returns nothing
  # pop() removes the top item from the stack.
  #    It needs no parameters and returns the item. The stack is modified.
  # peek() returns the top item from the stack but does not remove it.
  #    It needs no parameters. The stack is not modified.
  # isEmpty() tests to see whether the stack is empty.
  #    It needs no parameters and returns a boolean value.
  # size() returns the number of items in the stack.
  #    It needs no parameters and returns an int. 
  
  Class Stack:
      def __init__(self):
          self.items = []
  
      def isEmpty(self):
          return self.items == []
  
      def push(self,item):
          self.items.append(item)
  
      def pop(self):
          return self.items.pop()
  
      def peek(self):
          return self.items[len(self.items)-1]
      
      def size(self):
          return len(self.items)
  
subject: initialize a stack