Function for determining the number of documents

[xatapult]

In one of the things I did in XProc 3 I had a p:for-each that could spit out zero or more documents. If it were zero documents I needed some special handling, so I had to write a test for it. That is currently not straightforward. What I finally did was:

<p:if collection="true" test="count(collection()) eq 0">

That works but feels like a nerdy trick. I would advocate to add a function that returns the number of documents: p:document-count() or whatever.

Can also be solved by #48.