sequence stack = {}

procedure push(object what)
    stack = append(stack,what)
end procedure

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

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

?empty()                -- 1
push(5)
?empty()                -- 0
push(6)
?pop()                  -- 6
?pop()                  -- 5
?empty()                -- 1