[Feature request] Extend a monadic idiom u&.(m&{) to dyadic valence

[jip]

An idiom u&.(m&{) is monadic only implementing:

m}&y u m&{ y           NB. u&.(m&{) y

I propose to extend it to support dyadic application implementing:

m}&y (m&{ x) u m&{ y    NB. x u&.(m&{) y

That is, to make an idiom u&.(m&{) ambivalent.

[HenryHRich]

Not a bad thought, but the vast majority of uses would not need (m{x) so it would be better to use x instead. This is a semidual rather than a dual.

This is structural under, which is already a separate case of &. using its own inverse. Perhaps the dyad case of structural under should be a semidual.