test for presence of library

[Harry Putnam]

Can one test for the presence of a library before requiring it?

I mean in the way one might test for the presence of a file in common
shell script; if [ -f /my/file ];then
                  blab blab
              fi

Only here we are testing for the presence of a library somewhere in
the load path rather than a specific file location.

The lisp equivalent of:

 if some-lib 
 then
    (require some-lib)
 fi