bug#13422: 24.2.91; Feature request: Add a macro always expanding to nil

[Stefan Monnier]

> it has several advantages:

Of course, it also has its down sides:

- returns nil, so (if foo (discard bar) then) runs the `then' in the
  `else' branch.

- only works at those syntactic places where an expression is expected
  (e.g. (let ((a 1) (discard exp1 exp2) (b 2)) blabla) doesn't work.

In any case, I think the current comments are good enough, and save the
programmer from having to choose which kind of comment to use.


        Stefan