How do you define a new texinfo macro?

[Jordi Gutiérrez Hermoso]

We have a tiny problem with the docstring for mldivide and ldivide.
It's using backslashes in a the xcode macro, and texinfo has problems
with that:

    http://old.nabble.com/backslashes-in-macro-arguments-td28884883.html

A suggested workaround is to define a new macro just for backslashes
instead of passing backslashes as macro arguments. I was unable to
figure out how to do this. I tried monkeying the xcode macro, but I
was unable to make it work, so I thought I'd just ask.

TIA,
- Jordi G. H.