Allow for stems in intersection and union

[jjg-123]

Set union and intersection are, of course, useful. Now to do this on stems requires converting to sets first. Allow users to skip this by allowing the operations on stems with the proviso that the result is a set.

[jjg-123]

Note: The technical point is that the result of such an operation should be a set since otherwise the result is ambiguous. For instance, this gives a plausible intersection of stems a. and b.:

a.∈b.⌆a.

and returns the intersection in a. A similar expression for b. is possible but the point is that in general

a.∈b.⌆a. ≠ b.∈a.⌆b.

hence the only real way to get uniqueness is not to preserve the stem, but simply return the set of values that are common. Similar logic holds for the set union of two lists, since a. ∪ b. ≡ a.~b. ≠ b. ~a. ≡ b.∪ a. so in cases where a stem is required on return, the user has to actually manage the structure they want.