code: func (f *Filter) MatchEnvelope(envelope *Envelope) bool {
  	return f.PoW <= 0 || envelope.pow >= f.PoW
  }
docs: // MatchEnvelope checks if it's worth decrypting the message. If
  // it returns `true`, client code is expected to attempt decrypting
  // the message and subsequently call MatchMessage.
  // Topics are not checked here, since this is done by topic matchers.