Reported by Nikolaus Schaller, Jan 14, 2014

Which of the following methods is 'basic' and forwarded to the peers 
and which is 'derived'?

It appears that DO always uses a remote methodDescriptionForSelector 
even to get a methodSignatureForSelector, so we must implement this 
method for local objects (so that a client can ask us).

But we must base it in top of - (NSMethodSignature *) 
methodSignatureForSelector:(SEL) aSelector because that one might be 
overwritten to implement -forwardInvocation (see documentation 
there).

So clear answer: 
* -methodSignatureForSelector: is the basic (and user-visible) 
method
* and we just must be prepared that a distant object might ask for 
-methodDescriptionForSelector:
* and we must translate the NSMethodSignature into a struct 
objc_method_description *