code: func mutualProtocol(protos, preferenceProtos []string) (string, bool) {
  	for _, s := range preferenceProtos {
  		for _, c := range protos {
  			if s == c {
  				return s, false
  			}
  		}
  	}
  
  	return protos[0], true
  }
docs: // mutualProtocol finds the mutual Next Protocol Negotiation or ALPN protocol
  // given list of possible protocols and a list of the preference order. The
  // first list must not be empty. It returns the resulting protocol and flag
  // indicating if the fallback case was reached.