A stack is a linear data structure that store items in a Last-In/First-Out

class Stack:
	#assume __init__ and push(param1)

	def pop(self):
		if self._top != None
			self._top = self._top.next
		else:
			raise RuntimeError('Stack is empty')

	def peek(self):
		if self._top != None:
			return (self._top.entry)
		else:
			raise RuntimeError('Stack Empty')

	def is_empty():
		if self._top.next == None
			return (False)
		else:
			return (True)