XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (367 page)

Another way of solving this problem would be to write:

but I think the solution using the
is
operator is likely to be more efficient. (The
except
operator is described in Chapter 9, on page 628.)

The operators
<<
and
>>

The operators
<<
and
>>
test whether one node is before or after another in document order. For example,
$A << $B
is true if and only if
$A
precedes
$B
in document order. The concept of document order is described in Chapter 2, on page 57.