ometa RPN <: Parser {
  sign = '+' -> true
       | '-' -> false
       |     -> true,

  nonzero_digit = '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9',

  digits = nonzero_digit:firstDigit digit*:digits -> parseInt([firstDigit].concat(digits).join('')),
        
  number = '0' -> 0
         | sign:isPos digits:digits -> (isPos ? digits : -digits),

  numberTok = token(#number),

  binop = "+" -> (function(x, y) { return x + y; })
        | "-" -> (function(x, y) { return x - y; })
        | "*" -> (function(x, y) { return x * y; })
        | "/" -> (function(x, y) { return x / y; }),

  expr = token(#number):n -> n
       | binop:opfn expr:e1 expr:e2 -> opfn(e1, e2)
}

RPN.matchAll('123', 'expr')

// just thought I'd give you a little push... -Alex