what does \e do in :s/./\e&/ in vi? (was: merging 2 files)

[Kevin Kells]

In article <1990May5.193350.3445 at chinet.chi.il.us> les at chinet.chi.il.us 
(Leslie Mikesell) writes:
>   :%s/.*/& \L&/
>will do exactly what you ask, 

A little experimentation (didn't use source) proved that there are at
least three such \? "commands" that can be used in this substitution
context. Two I understand: \l (not \L) and \u, which convert upper case
to lower case and vice versa, respectively.  But a third, \e, also
seems to do something, though I can't figure out what.

Is this documented somewhere [besides in the source :-) ] ? Could
someone please tell me what \e does in this context?

Thanks,

Kevin