[m-rev.] for review: priority search queue ADT

[Matthias Güdemann]

Hi Paul

> I've adding this to Mercury at the moment.  Do you have some test code
> that I can commit to the test suite?

yes, I have some small test cases. I will have some time tomorrow to add
them in a suitable format.

> I'm very sorry for making this drag on.  I'll commit the code now and
> fix up the few minor issues with a commit of my own.

No problem, I am happy to contribute and to see that the code is useful
for the project.

Matthias