Nearby items in a list
x xprev y xprev[x;y]
Where x is a long atom and y is a list, returns for each item of y the item
x indices before it.
The first x items of the result are null, empty or blank as appropriate.
There is no xnext function.
Fortunately xprev with a negative number on the left can achieve this.
q)2 xprev 2 7 5 3 11
0N 0N 2 7 5
q)-2 xprev 2 7 5 3 11
5 3 11 0N 0N
q)1 xprev "abcde"
" abcd"
xprev is a right-uniform function.