code: function(a, b, total) {
        if (a == 0) return b > 0 ? [b] : [];
        return $R(1, total).inject([], function(memo, i) {
          if (0 == (i - b) % a && (i - b) / a >= 0) memo.push(i);
          return memo;
        });
      }
docs: handles the an+b logic