What sets textwidth=78

Using Huge vim 7.4 with patch 1-473 on arch linux

One of the last entries in my .vimrc is

  set textwidth=0

which is occasionally overridden by some feature in vim that arbitrarily sets the textwidth to 78.

Most of the time this creates no problem, but when I happen to add text to a .vimrc mapping  that exceeds 78 characters, arbitrary line breaks are introduced into it, which matters a great deal.

Does anyone know what might do this?

--

Graham Lawrence

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