Referential transparency on non-deterministic functions

[AnnelineD]

Describe the bug
Non-deterministic functions are not referentially transparent.

To Reproduce

> !(subtraction (superpose (a b d)) (intersection (superpose (a b c)) (superpose (a b d))))
[d]
> (= (test6 $set $subset) (subtraction $subset (intersection $set $subset)))
> !(test6 (superpose (a b c)) (superpose (a b d)))
[b, d, a, d, a, b, d]

Expected behavior
The wrapped function test6 produces the same result as the top-level test.

Actual behavior
test6 gets applied element-wise.

Additional context

• MeTTa version: 0.1.10