Please say why there is an unused optional arg ARG.

In GNU Emacs 26.1 (build 1, x86_64-w64-mingw32)

of 2018-05-30

Repository revision: 07f8f9bc5a51f5aa94eb099f3e15fbe0c20ea1ea

Windowing system distributor `Microsoft Corp.', version 10.0.16299

Configured using:

`configure --without-dbus --host=x86_64-w64-mingw32

--without-compress-install 'CFLAGS=-O2 -static -g3''