Even when Forrest Sawyer is sitting in, it's still World News Tonight With Peter Jennings . Just because Johnny goes on vacation, The Tonight Show Starring Johnny Carson doesn't suddenly become The Tonight Show With Jay Leno . The task of a guest host is delicate.
Johnny Carson went on vacation often thus giving Jay Leno something to do.  
mnli hypothesis: Johnny Carson went on vacation often thus giving Jay Leno something to do.   premise: Even when Forrest Sawyer is sitting in, it's still World News Tonight With Peter Jennings . Just because Johnny goes on vacation, The Tonight Show Starring Johnny Carson doesn't suddenly become The Tonight Show With Jay Leno . The task of a guest host is delicate.