public interface Unstable
Marker interface for a function that is "unstable". That is, it isn't really a function.
A function returns the same result value when called with the same arguments. Some "functions" do not obey this condition on functions; they look like functions but aren't. RAND, BNODE, UUID, STRUUID
There are also "functional forms" that have special evaluation rule. These return the same value when given the same arguments. These are "stable".