Conditional evaluation?

[Elena]

Hi,

whenever I'm testing a new chunk of Emacs Lisp, I'd like to
conditionally include/exclude it without resorting to "comment-
region". Is using "when" correct?

    (when nil ;; use t to enable evaluation
        (require 'some-package)
        ;; etc.
    )

Or do I need an alternative form to make code evaluate at top level?

Thanks.