Simple messages problem

FROM : K. Darcy Otto
DATE : Thu Mar 20 23:25:34 2008

So, I seem to be having a problem with the following code, and I think 
I've traced it to how the messages are nested; but I cannot seem to 
solve it.  Consider the following code, which works without warnings 
(self.sequent is an NSArray):

   DeductionLine *dl1 = [self.sequent objectAtIndex:lineCount-1];
   Justification *j1 = [dl1 justification];
   NSString *rule1 = [j1 rule];
   
But the following line generates a warning: NSString may not respond 
to -rule.

   NSString *rule2 = [[[self.sequent objectAtIndex:lineCount-1] 
justification] rule];

I'm confused about the warning, because [[self.sequent 
objectAtIndex:lineCount-1] justification] returns type Justification, 
not NSString.  Both examples compile and work; but the warning is in 
the second case only.  Why, and how can I get rid of the warning?  Any 
help would be appreciated.  (I think I've included everything 
necessary to diagnose the problem; please let me know if I haven't; 
all of this is under Objective-C 2.0 with @synthesize sequent in 
DeductionLine.)