code: func (header *tokenHeader) valid() bool {
  	// RFC7515 s4.1.1 alg MUST be present
  	if header.alg == "" {
  		return false
  	}
  	// RFC7515 4.1.11 JWS is invalid if there is a critical parameter that we did not recognize
  	for _, u := range header.unknown {
  		if header.crit[u] {
  			return false
  		}
  	}
  	return true
  }
docs: // validTokenHeader returns true if the JOSE header is valid, otherwise false.