The positions selected by x u} y may be made to depend on
either or both of the arguments x and y ,
and related adverbs can be defined for convenient use in common cases.
For example:
A=: @(i.@$@])
u=: (<0 1)&|:
x=: 'DIAG' [ y=: a. {~ (a. i. 'a') + i. 4 5
x;y;(x u A y);(x u A} y)
+----+-----+---------+-----+
|DIAG|abcde|0 6 12 18|Dbcde|
| |fghij| |fIhij|
| |klmno| |klAno|
| |pqrst| |pqrGt|
+----+-----+---------+-----+