code: func (wc *WhisperClient) NewMessageFilter(ctx *Context, criteria *Criteria) (string, error) {
  	return wc.client.NewMessageFilter(ctx.context, *criteria.criteria)
  }
docs: // NewMessageFilter creates a filter within the node. This filter can be used to poll
  // for new messages (see FilterMessages) that satisfy the given criteria. A filter can
  // timeout when it was polled for in whisper.filterTimeout.