code: func NewEncryptStream(version Version, ciphertext io.Writer, sender BoxSecretKey, receivers []BoxPublicKey) (io.WriteCloser, error) {
  	ephemeralKeyCreator, err := receiversToEphemeralKeyCreator(receivers)
  	if err != nil {
  		return nil, err
  	}
  	return newEncryptStream(version, ciphertext, sender, receivers, ephemeralKeyCreator, defaultEncryptRNG{})
  }
docs: // NewEncryptStream creates a stream that consumes plaintext data.
  // It will write out encrypted data to the io.Writer passed in as ciphertext.
  // The encryption is from the specified sender, and is encrypted for the
  // given receivers.
  //
  // If initialization succeeds, returns an io.WriteCloser that accepts
  // plaintext data to be encrypted and a nil error. Otherwise, returns
  // nil and the initialization error.