evalAddSub = function()
    result = evalMultDiv
    while true
        if not tokens then return result
        op = tokens[0]
        if op != "+" and op != "-" then return result
        tokens.pull  // (discard operator)
        rhs = evalMultDiv
        if result == null or rhs == null then return null
        if op == "+" then result = result + rhs
        if op == "-" then result = result - rhs
    end while
end function

evalMultDiv = function()
    result = evalAtom
    while true
        if not tokens then return result
        op = tokens[0]
        if op != "*" and op != "/" then return result
        tokens.pull  // (discard operator)
        rhs = evalAtom
        if result == null or rhs == null then return null
        if op == "*" then result = result * rhs
        if op == "/" then result = result / rhs
    end while
end function

evalAtom = function()
    if tokens[0] == "(" then
        tokens.pull
        result = evalAddSub
        if not tokens or tokens.pull != ")" then
            print "Unbalanced parantheses"
            return null
        end if
        return result
    end if
    num = val(tokens.pull)
    idx = availableDigits.indexOf(num)
    if idx == null then
        print str(num) + " is not available"
        return null
    else
        availableDigits.remove idx
    end if
    return num
end function

choices = []
for i in range(1, 4)
    choices.push ceil(rnd*9)
end for
result = null
while result != 24
    availableDigits = choices[:]  // (clones the list)
    print "Using only the digits " + availableDigits + ","
    tokens = input("enter an expression that comes to 24: ").replace(" ","").values
    result = evalAddSub
    if availableDigits then
        print "You forgot to use: " + availableDigits
        result = null
    end if
    if result != null then print "That equals " + result + "."
end while
print "Great job!"