Finding variables by their value

[Jim Crossley]

I thought I remembered seeing somewhere that it was possible to search
for lisp variables that had a particular value.  Anyone know if that's
possible?

Like, say my w3m can't get to http://localhost/blah because it's
using a proxy, and I know the hostname of the proxy, so I want to find
the variable that contains the proxy so I can unset it.  What's the
command that says, "Show me the variable whose value contains X".

Thanks,
Jim