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.
In the twoargument case, returns:
$sourceSeq[fn:round($startingLoc) le $p]
In the threeargument case, returns:
$sourceSeq[fn:round($startingLoc) le $p and $p lt fn:round($startingLoc) + fn:round($length)]
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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