In both cases it seems as though $a contains only 1 thing, a
string. (You can test that it is not a node set by using $a in
the select= of a for-each -- you should get a runtime error,
since it is a string.)

No, it's a result tree fragment, not a string.

Oh, right. Thanks for the correction. (He says with his tail between
his legs.) But if I understand correctly, as a result tree fragment,
only those operations that can be performed on strings are permitted
(i.e., can't do path stuff or predicates), and you can't use a result
tree fragment as the value of select= of for-each (or apply-
templates, for that matter), because a result tree fragment is not a
node-set, even though when you do get to use one it behaves as node-
set. (And select= of for-each and apply-templates takes a node-set.)

exslt:node-set (and xalan:nodeset, saxon;node-set, msxsl:node-set, etc)
work with result tree fragments not strings.

________________________________________________________________________
The Numerical Algorithms Group Ltd is a company registered in England
and Wales with company number 1249803. The registered office is:
Wilkinson House, Jordan Hill Road, Oxford OX2 8DR, United Kingdom.