RFC: FOO_subtract, the logical antidote to FOO_append.

[Peter Seebach <peter.seebach@windriver.com>]

There's a few cases where something is a huge list of space-separated
things, and it is desireable to remove one.  The example currently
afflicting me is DISTRO_FEATURES_LIBC_DEFAULT; I want to end up with
the distro features including all but one of the words in it.

It seems to me that a counterpart to _append would make sense.  Here
is my basic idea:

FOO_subtract = "..."

means that, when you expand FOO:

1. Fully expand it.
2. Fully expand FOO_subtract.
3. Remove any words in FOO_subtract from FOO.
4. Yield the result.

The rationale is that the semantics of things where we're using _append
seem to be consistently of the form "this is a space-separated set",
and being able to remove things from a set would be Super Handy.

So I'm proposing the semantics for consideration, and if people like
them, I will go try to implement it in my Copious Free Time.

-s
-- 
Listen, get this.  Nobody with a good compiler needs to be justified.