code: func (c *Capability) GenerateToken(ttl time.Duration) (string, error) {
  	c.doBuildIncomingScope()
  	c.doBuildOutgoingScope()
  	now := time.Now().UTC()
  	cc := &customClaim{
  		Scope: strings.Join(c.capabilities, " "),
  		StandardClaims: &jwt.StandardClaims{
  			ExpiresAt: now.Add(ttl).Unix(),
  			Issuer:    c.accountSid,
  			IssuedAt:  now.Unix(),
  		},
  	}
  	token := jwt.NewWithClaims(jwt.SigningMethodHS256, cc)
  	return token.SignedString([]byte(c.authToken))
  }
docs: // Generate the twilio capability token. Deliver this token to your
  // JS/iOS/Android Twilio client.