fn:insert-before( $target as item()*, $position as xs:integer, $inserts as item()* ) as item()*
Returns a new sequence constructed from the value of $target with the value of $inserts inserted at the position specified by the value of $position. (The value of $target is not affected by the sequence construction.)
If $target is the empty sequence, $inserts is returned. If $inserts is the empty sequence, $target is returned.
The value returned by the function consists of all items of $target whose index is less than $position, followed by all items of $inserts, followed by the remaining elements of $target, in that sequence.
If $position is less than one (1), the first position, the effective value of $position is one (1). If $position is greater than the number of items in $target, then the effective value of $position is equal to the number of items in $target plus 1.
For detailed semantics see, Section 7.2.15 The fn:insert-before function[FS].
let $x := ("a", "b", "c") return fn:insert-before($x, 0, "z") returns ("z", "a", "b", "c") let $x := ("a", "b", "c") return fn:insert-before($x, 1, "z") returns ("z", "a", "b", "c") let $x := ("a", "b", "c") return fn:insert-before($x, 2, "z") returns ("a", "z", "b", "c") let $x := ("a", "b", "c") return fn:insert-before($x, 3, "z") returns ("a", "b", "z", "c") let $x := ("a", "b", "c") return fn:insert-before($x, 4, "z") returns ("a", "b", "c", "z")