[m-dev.] for review: add `do_while/4' to std_util.m

[Ralph Becket]

Just one point...

> :- pred q(int::out, int::out) is nondet.
> 
> q(X, Y) :-
> 	p(X),
> 	(if some [Y1] p(Y1) then
> 		Y = Y1
> 	else
> 		Y = 42
> 	).

I find the quantification here a tad confusing.  My intuition says that
the quantification `some [Y1]' has scope just in the condition of the
if-then-else - which would make `Y1' free in the then clause - but it
clearly extends into the then clause.  I would write this along the
lines of

	Y = some [Y1] (if p(Y1) then Y1 else 42)

I know it's just some test code; I just want to know if Fergus' code is
the `correct' idiom and, if so, why?

Ta,

Ralph
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