on keyDown k
    local ops, nums, allowedKeys, numsCopy, expr
    put "+,-,/,*,(,)" into ops
    put the text of fld "YourNumbersField" into nums
    put the text of fld "EvalField" into expr
    if matchText(expr & k,"\d\d") then
        answer "You can't enter 2 digits together"
        exit keyDown
    end if
    repeat with n = 1 to the number of chars of expr
        if offset(char n of expr, nums) > 0 then
            delete char offset(char n of expr, nums) of nums
        end if
    end repeat
    put ops & comma & nums into allowedKeys
    if k is among the items of allowedKeys then
        put k after expr
        delete char offset(k, nums) of nums
        replace comma with empty in nums
        try
            put the value of merge("[[expr]]") into fld "AnswerField"
            if the value of fld "AnswerField" is 24 and nums is empty then
                answer "You win!"
            end if
        end try
        pass keyDown
    else
        exit keyDown
    end if
end keyDown