Question on proposed use of "svn switch"

[Richard Kenner]

I just finished reading the SVN manual. This was the 1.1 version, so I hope
there's nothing important in the newer versions.

Somebody at one point asked how to have a subdirectory of the repository
that wouldn't be touched so they could have something entirely different
in it.  I think "svn switch" was suggested, but I can't find that message.
I recall it being some URL like file:/dev/null, but when I read the manual,
I find that the operand of "svn switch" needs to be in the same repository
as the rest of the working directory, so I'm confused.
	
Here's what I need to do and I welcome suggestions: one of the working
directories I have is the FSF GCC repository (from HEAD), but the
gcc/ada subdirectory is the AdaCore repository.  For cvs, what I do in
gcc/CVS/Entries is delete the line for "ada" and then checkout the AdaCore
repository into there.  What's the way to do this in svn?