Type assertions for party-collapsing types

[Zettelkasten]

Consider you have some partly-collapsing variable x: Ref[A]|B which through reference collapsing always acts as A|B.

Assume we want to check / assert that x actually is a Ref[A]. This is currently impossible, because in if x is Ref[A] { ... } also x acts as A|B, so this would in general by false.
The same goes for assert(x is Ref[A]).

Instead, you must check for if x is A { ... }.
This doesn't work for x: Ref[A]|A though.

A (ugly) workaround is to use the complementary check, i.e. if not(x is B) { ... }.