RPC(
  query,
  INT32_FIELD(msg_len)
  INT32_FIELD(request_id)
  INT32_FIELD(response_to)
  INT32_FIELD(opcode)
  ENUM_FIELD(flags)
  CSTRING_FIELD(collection)
  INT32_FIELD(skip)
  INT32_FIELD(n_return)
  BSON_FIELD(query)
  BSON_OPTIONAL(fields, BSON_FIELD(fields))
)