configuration.go
func PrivateKey(value crypto.Signer) Option {
	return func(c *configuration) {
		c.priv = &value
	}
}
func PrivateKey(value crypto.Signer) Option {
	return func(c *configuration) {
		c.priv = &value
	}
}
// PrivateKey is an Option for setting the identity's private key.