def nth_child_production(self, lexeme, tokens):
        """Parse args and pass them to pclass_func_validator."""

        args = self.match(tokens, 'expr')

        pat = self.nth_child_pat.match(args)

        if pat.group(5):
            a = 2
            b = 1 if pat.group(5) == 'odd' else 0
        elif pat.group(6):
            a = 0
            b = int(pat.group(6))
        else:
            sign = pat.group(1) if pat.group(1) else '+'
            coef = pat.group(2) if pat.group(2) else '1'
            a = eval(sign + coef)
            b = eval(pat.group(3) + pat.group(4)) if pat.group(3) else 0

        reverse = False
        if lexeme == 'nth-last-child':
            reverse = True

        def validate(node):
            """This crazy function taken from jsonselect.js:444."""

            if not node.siblings:
                return False

            idx = node.idx - 1
            tot = node.siblings

            if reverse:
                idx = tot - idx
            else:
                idx += 1

            if a == 0:
                m = b == idx
            else:
                mod = (idx - b) % a
                m = not mod and (idx * a + b) >= 0
            return m

        return validate