fn.subsequence( sourceSeq as Sequence, startingLoc as Number, [length as Number] ) as Sequence
Returns the contiguous sequence of items in the value of $sourceSeq
beginning
at the position indicated by the value of $startingLoc
and continuing for the
number of items indicated by the value of $length
.
As described in https://www.w3.org/TR/xpathfunctions/:
In the twoargument case, fn.subsequence
returns a sequence comprising those items of
$sourceSeq
whose index position (counting from one) is greater than or equal to the
value of $startingLoc
(rounded to an integer):
$sourceSeq[fn:round($startingLoc) le $p]
No error occurs if $startingLoc
is zero or negative.
In the threeargument case, The function returns a sequence comprising those items of
$sourceSeq
whose index position (counting from one) is greater than or equal to the
value of $startingLoc
(rounded to an integer), and less than the sum of
$startingLoc
and $length
(both rounded to integers):
$sourceSeq[fn:round($startingLoc) le $p and $p lt fn:round($startingLoc) + fn:round($length)]
No error occurs if $startingLoc
is zero or negative, or if $startingLoc
plus $length
exceeds the number of items in the sequence, or if
$length
is negative.
Notes:
If $sourceSeq is the empty sequence, the empty sequence is returned.
If $startingLoc is zero or negative, the subsequence includes items from the beginning of the $sourceSeq.
If $length is not specified, the subsequence includes items to the end of $sourceSeq.
If $length is greater than the number of items in the value of $sourceSeq following $startingLoc, the subsequence includes items to the end of $sourceSeq.
The first item of a sequence is located at position 1, not position 0.
For detailed type semantics, see Section 7.2.13 The fn:subsequence functionFS.
The reason the function accepts arguments of type xs:double is that many computations on untyped data return an xs:double result; and the reason for the rounding rules is to compensate for any imprecision in these floatingpoint computations.
Parameters  

sourceSeq 
A Sequence of items from which a subsequence will be
selected. If you pass in a single value,
it is treated as a Sequence with that single item; therefore, if you
pass in an array, fn.subsequence will treat the array as
a singleitem sequence.
If you mean to get the subsequence of each item in the array, then
you can call xdmp.arrayValues
on the array.

startingLoc  The starting position of the start of the subsequence. 
length  The length of the subsequence. 
const seq = xdmp.arrayValues(["item1", "item", "item3", "item4" , "..."]); fn.subsequence(seq, 4) => (item4, ...) const seq = xdmp.arrayValues(["item1", "item", "item3", "item4" , "..."]); fn.subsequence(seq, 3, 2) => (item3, item4)
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