Stacks

Stack S = x0  x1  x2  x3 ... xn-1     n =  # elements
A stack is like a list but the nodes are only accessed last-in-first-out
or LIFO.

Functions:
createEmptyStack()	returns a newly created empty stack
top(S)   		returns the last node of S
pop(S)  		returns and removes the last node of S
push(S, x)		returns a S with x added as the last element 
isEmptyStack(S)		returns true if