code: func NewWebRegisterRequest(c *Challenge, regs []Registration) *WebRegisterRequest {
  	req := RegisterRequest{
  		Version:   u2fVersion,
  		Challenge: encodeBase64(c.Challenge),
  	}
  
  	rr := WebRegisterRequest{
  		AppID:            c.AppID,
  		RegisterRequests: []RegisterRequest{req},
  	}
  
  	for _, r := range regs {
  		rk := getRegisteredKey(c.AppID, r)
  		rr.RegisteredKeys = append(rr.RegisteredKeys, rk)
  	}
  
  	return &rr
  }
docs: // NewWebRegisterRequest creates a request to enrol a new token.
  // regs is the list of the user's existing registration. The browser will
  // refuse to re-register a device if it has an existing registration.