code: func (mgr *MockIdentityManager) GetSigningIdentity(id string) (msp.SigningIdentity, error) {
  	si, ok := mgr.users[id]
  	if !ok {
  		return nil, msp.ErrUserNotFound
  	}
  	return si, nil
  }
docs: // GetSigningIdentity will return an identity that can be used to cryptographically sign an object