[Jprogramming] Feature proposal: structural under

[Elijah Stone]

'Structural under' is an extension to under (&. &.:) that allows it to 
operate on otherwise uninvertible structure-changing verbs.  It is 
implemented in dzaima/apl (https://github.com/dzaima/APL/).

For example:

    -&.{. 1 2 3 4  NB. take the head, negate it, reaffix it to the rest of 
the body
_1 2 3 4
    }:&.}. 1 2 3 4 NB. remove the head, remove the tail, and then sew the 
head back on
1 2 3
    |:&.}. i.4 3   NB. remove the head, transpose the body, and then 
reattach the head
1 2  3
3 6  9
4 7 10
5 8 11
   |.&., i.2 3     NB. ravel, reverse, and then unravel
5 4 3
2 1 0

To make this functionality available to users, I suggest using .: for 
'assign under', by analogy to :. and ::.  Then:

   u&.:(v .: vu) y  ←→  vu y;u v y.
x u&.:(v .: vu) y  ←→  vu x;y;(v x) u v y

The reason that vu is only applied monadically is to allow for the 
expected identity:

u&.:(m&(v .: vu)) y ←→ m vu y;u v y

Thoughts?

  -E