selecting by multiple classes

[Chris Maden <crism@xxxxxxxxxxx>]

Further on the problem of comparing element and PI positions:

I implemented a modified (tree-before?) based only on the subtree in
which I was interested.  But it seems that the only way to get an
ordered list of nodes is to take the intersection of (subtree) and the
unordered node list, and (node-list-intersection) seems to be taking a
long time.  That makes sense, since (node-list-intersection) compares
every node in one list to every node in the other, and (subtree)
returns everything.

If I were just interested in elements, I could use (select-by-class)
to get just the element descendants of the context element; ditto if
it were just PIs.  Is there a way to do the equivalent of (select-by-
class) for more than one class, to help speed this up?  I can't just
build a node list out of two (select-by-class)s, because that will
lose the relative order which is the point of this exercise.

Thanks in advance,
Chris
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