* A "recognizer" for arithmetic expressions

    nl = char(10)

    expr = 'x'
    
loop
    line = input                :f(end)
    line pos(0) expr rpos(0)           :f(huh)

    output = 'ok: ' line nl     :(loop)
huh output = '??: ' line nl     :(loop)

end

* x
* abc
* abc+x
* a*b+c-d/e
* 10
* -20
* (a-1)
* -20*(3+4-(x*y))
* 3*(4+5)
* ((x+y)*z)/2+((x))-(q/3)*500                                            
* (x+3)*