XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (86 page)

, and it concatenates the two-item sequence
1, 2
with the one-item sequence
3
to produce the three-item sequence
1, 2, 3
. This definition of comma as an operator means that it is also possible to write sequences such as
$a, $b
, which concatenates two arbitrary sequences represented by the variables
$a
and
$b
.

Some of the important XPath operations defined on sequences are:

• count($S)
  counts the items in a sequence.
  
• $S, $T
  concatenates two sequences.
  
• $S[predicate]
  selects those items in a sequence that satisfy some condition.
  
• $S[number]
  selects the
  N
  th item in a sequence.
  
• for $item in $SEQ return f($item)
  applies the function
  f
  (which can actually be any expression) to every item in the sequence
  $SEQ
  , and returns the results as a new sequence. (In list processing languages, this is known as a
  mapping
  expression).