global stack$

expr$ = "3 4 2 * 1 5 - 2 3 ^ ^ / +"
print "Expression:"
print expr$
print

print "Input","Operation","Stack after"

stack$=""
token$ = "#"
i = 1
token$ = word$(expr$, i)
token2$ = " "+token$+" "

do
    print "Token ";i;": ";token$,
    select case
    'operation
    case instr("+-*/^",token$)<>0
        print "operate",
        op2$=pop$()
        op1$=pop$()
        if op1$=""  then
            print "Error: stack empty for ";i;"-th token: ";token$
            end
        end if

        op1=val(op1$)
        op2=val(op2$)

        select case token$
        case "+"
            res = op1+op2
        case "-"
            res = op1-op2
        case "*"
            res = op1*op2
        case "/"
            res = op1/op2
        case "^"
            res = op1^op2
        end select

        call push str$(res)
    'default:number
    case else
        print "push",
        call push token$
    end select
    print "Stack: ";reverse$(stack$)
    i = i+1
    token$ = word$(expr$, i)
    token2$ = " "+token$+" "
loop until token$ =""

res$=pop$()
print
print "Result:" ;res$
extra$=pop$()
if extra$<>"" then
    print "Error: extra things on a stack: ";extra$
end if
end

'---------------------------------------
function reverse$(s$)
    reverse$ = ""
    token$="#"
    while token$<>""
        i=i+1
        token$=word$(s$,i,"|")
        reverse$ = token$;" ";reverse$
    wend
end function
'---------------------------------------
sub push s$
    stack$=s$+"|"+stack$    'stack
end sub

function pop$()
    'it does return empty on empty stack
    pop$=word$(stack$,1,"|")
    stack$=mid$(stack$,instr(stack$,"|")+1)
end function