Markus Mottl <mottl@miss.wu-wien.ac.at> writes:

> Optional arguments can also be used to pass the "same" argument in
> different forms (by using different labels on them), as is the case in the
> PCRE: you can pass regular expressions as a string, in which case it will
> be compiled for you, or as a precompiled regular expression, useful if
> matching happens in a loop. This can be done by having two optional
> arguments, one overriding the other if present.  There is no commuting
> involved (not necessarily), but this "trick" still gives the user a great
> deal more of flexibility: he can choose whether he wants the "readable" or
> "efficient" form without having to memorize more function names.

Welcome to dynamic lisp.

Cheers,
Julian.