code: func prependSigningContext(blob []byte, ctx string) []byte {
  	if ctx == "" {
  		return blob
  	}
  	b := make([]byte, len(blob)+len(ctx)+1)
  	copy(b, ctx)
  	copy(b[len(ctx)+1:], blob)
  	return b
  }
docs: // prependSigningContext prepends '<ctx>\x00' to the blob, if ctx != "".
  //
  // See SigningContext in Signer for more info.