code: func Envelope(policy *cb.SignaturePolicy, identities [][]byte) *cb.SignaturePolicyEnvelope {
  	ids := make([]*msp.MSPPrincipal, len(identities))
  	for i := range ids {
  		ids[i] = &msp.MSPPrincipal{PrincipalClassification: msp.MSPPrincipal_IDENTITY, Principal: identities[i]}
  	}
  
  	return &cb.SignaturePolicyEnvelope{
  		Version:    0,
  		Rule:       policy,
  		Identities: ids,
  	}
  }
docs: // Envelope builds an envelope message embedding a SignaturePolicy