What type should NO_FIELDS be?

[oxinabox]

NO_FIELDS should be Zero() not DNE()

Thought about it.
The derviative with respect to all the fields of which there is none,
is probably better decribed as Zero.
There is no relation between those (onexistant) fields and the function output.

And we don't want it to error when externed.

[oxinabox]

IDK, maybe it should be Composite ?

[oxinabox]

It should be Zero()
because simplify(Composite{Primal}(a=Zero())) = Zero())
as is simplify(Composite{Primal}()) = Zero())

[nickrobinson251]

Can you give some more references for this? I do not understand the comment above.

[oxinabox]

Notionally, we could have the idea of being able to simplify a Composite type.
This would mean dropping any explict Zero() fields,
since we always treat missing fields as Zero()
Further, if we got down the a Composite{Primal}() we could simplify that the Zero() since if added to the primal it would not change anything since all fields are treated as Zero().

And in the case of some object or function of type T without any fields,
Its composite type would naturally be Composite{T}().
But since we don't want to go to the effort to create these objects, we can just user Zero().