Surely the point of specifying a starting node would be to operate
on a sub-document so that in your example /part/book/index would
be invisible to the XSLT processing. Your 'equivalent' syntax requires
the whole document to exist in memory.
XSLT 1.0 didn't allow a transformation to start at any node, but MSXML3 and
JAXP both did, though both were a bit vague about the semantics. In both
cases the starting node is not constrained to be the root of a tree. If the
calling API wants to give you the ability to carve out a subtree and supply
the root node of that subtree as the place where transformation should
start, then it can do so, but that's outside the XSLT scope. Calling "/" in
XSLT at the outermost level (e.g. in a global variable) gives you the root
of the document containing the initial context node.
Michael Kay