der Mouse is right, IMHO.  While it may be *convenient* to have programs
(such as "compress/uncompress") which mutate, it is very important to be
able to specify this behavior with flags, too; at the very least, it is
convenient.

Why?  Say I want to have local copies, named 'scompress' and 'suncompress'.
It won't work.  Either I build two binaries with different rules, or I learn
to say 'scompress -d'.  That '-d' (or however compress spells it) makes it
possible for me not to have one *different* binary for each behavior.

The GNU coding standards cover this, too, and it's one of the cases where
they stated the argument well.

-s