function push(sequence stack, object what)
    stack = append(stack,what)
    return stack
end function

function pop(sequence stack)
    object what = stack[$]
    stack = stack[1..$-1]
    return {stack,what}
end function

function empty(sequence stack)
    return length(stack)=0
end function

sequence stack = {}
?empty(stack)               -- 1
stack = push(stack,5)
?empty(stack)               -- 0
stack = push(stack,6)
integer top
{stack,top} = pop(stack)
?top                        -- 6
{stack,top} = pop(stack)
?top                        -- 5
?empty(stack)               -- 1