unclear on shared lock support

[John Meissen]

RFC4918 says

6.3.  Required Support

   A WebDAV-compliant resource is not required to support locking in any
   form.  If the resource does support locking, it may choose to support
   any combination of exclusive and shared locks for any access types.

To simplify life I chose to only implement exclusive locks. It's not
clear to me what effect that has on requests for shared locks.

I consider shared vs exclusive to be a lock attribute. There are other
requested attributes, like timeout, that a server is free to ignore.
I interpreted the RFC as allowing me to ignore the request to make the
lock shared, and permitting me to create the lock in the mode I support
(exclusive), especially since the actual lock characteristics are 
detailed in the response body. Hence there shouldn't be any confusion
on the client's part since it can see what type of lock was created.

The lock tests in litmus make the assumption that shared locks are
supported. It's not clear to me whether litmus expects me to return
an error if shared locks aren't supported, or if it's just blindly
assuming shared lock capability. If it's the latter, it should 
retrieve the DAV:supportedlock property first to determine if that's
a valid assumption. If it's the former, is my interpretation of the
RFC wrong?

Constructive feedback greatly appreciated.

john-