How to know which RPM has a file?

[Chris Hoelscher]

I know how to tell what files are in an RPM package, but when I try
loading Opera RPM on my RH 7.3, it complains that libXm.so is needed. 
How can I determine what RPM provides that?  I've gone to RPMFIND and
such and searching for 'libXm' provides a number of 'libxml-etc'
pacakges, but trying a couple of them does not seem to satisfy.

Any ideas?  Tricks?

Thanks -
Chris H.