code: public static Query withField(String field,
                                    BinOp op,
                                    String rfield) {
          Query q = new Query(false);
          q.add("field", field).add("op", op.toString()).add("rfield", rfield);
          return q;
      }
docs: <pre>
  { field: <field>, op: <op>, rfield: <rfield> }
  </pre>