set subtract

[byron@netapp.com (Byron Rakitzis)]

Given two lists a and b, it is possible to do set subtract
in rc with the ~ operator:

	for (i in $a)
		if (!~ $i $b)
			echo $i

So all that you need to do is write a function to build
these two lists based on the

	[a] - [b]

syntax suggested. Something like this:

	fn set-subtract {
		a=()
		while () {
			switch ($1) {
			case ()
				echo usage: $0 'foo - bar' >[1=2]
				return 1
			case -
				shift
				break
			case *
				a=($a $1)
				shift
			}
		}
		for (i in $a)
			if (!~ $i $*)
				echo $i
	}

(Btw, I would choose a shorter name than set-subtract. Sort of reminds
me of the regexp syntax that Nicklaus Wirth uses in his Oberon system.
I think it's straight BNF.  To say .* you have to write it as {~c}. Or
even worse, foo*bar becomes "fo"{"o"}"bar". Does he really use this
tool, or expect anyone else to? But I digress..)