Ann: delay pack

I've just released a pack which helps avoid instantiation errors with
built-in predicates.  By using delay/1 a single clause can operate in many
modes without manually re-ordering goals.  You can read the full
documentation <http://www.swi-prolog.org/pack/list?p=delay> for details.
 Here's a quick demo:

?- delay(succ(A,B)), A=1.
A = 1,
B = 2.

?- delay(plus(A,3,C)), C=5.
A = 2,
C = 5.

?- delay(univ(T,Name,Args)), Name=hello, Args=[world].
T = hello(world),
Name = hello,
Args = [world].

I finally got tired enough of writing the same when/2 goals over and over
that I figured it was time for a library.  I find this style especially
helpful with DCGs where it allows predicates to both generate and parse
content with a single definition.

As always, feedback, criticism and pull requests are welcome.

-- 
Michael
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