* A "recognizer" for arithmetic expressions

    nl = char(10)
	
    var = span(&lcase)
    op = any('+-/*')
    int = span(&digits)
    expr = int | var | *expr op *expr | '-' *expr | '(' *expr ')'
    
loop
    line = input                :f(end)
    line pos(0) expr rpos(0)    :f(huh)

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

end