code: func (id *MockIdentity) Verify(msg []byte, sig []byte) error {
  	if id.Err != nil && id.Err.Error() == "Verify" {
  		return id.Err
  	}
  	return nil
  }
docs: // Verify checks against a signature and a message
  // to determine whether this identity produced the
  // signature; it returns nil if so or an error otherwise