Sad news. Belson was one of the best comedy writers around, both for scripts and for uncredited punch-ups, which was his main job in the last couple of decades. (For example, James L. Brooks hired him to do some uncredited work on the first flashback episode of The Simpsons.) I wrote a little bit about him last December, as did Ken Levine.
Here's a chunk of a Dick Van Dyke episode that Belson and his (then) writing partner Garry Marshall wrote together: