sequence stacks = {}
integer freelist = 0

function new_stack()
integer res = freelist
    if res!=0 then
        freelist = stacks[freelist]
        stacks[res] = {}
    else
        stacks = append(stacks,{})
        res = length(stacks)
    end if
    return res
end function

procedure free_stack(integer sid)
    stacks[sid] = freelist
    freelist = sid
end procedure

procedure push(integer sid, object what)
    stacks[sid] = append(stacks[sid],what)
end procedure

function pop(integer sid)
    object res = stacks[sid][$]
    stacks[sid] = stacks[sid][1..$-1]
    return res
end function

function empty(integer sid)
    return length(stacks[sid])=0
end function

integer sid = new_stack()
?empty(sid)                 -- 1
push(sid,5)
?empty(sid)                 -- 0
push(sid,6)
?pop(sid)                   -- 6
?pop(sid)                   -- 5
?empty(sid)                 -- 1
free_stack(sid)