Overlap operators for ARRAY and NULLs

There might be inconsistency in overlap operators for array
types ( <@, @>, and && ) when the argument arrays contain NULLs.

- SELECT 2 = ANY (ARRAY[1, NULL]) => NULL
- SELECT ARRAY[2] <@ ARRAY[1, NULL] => false

NULL means "unknown" in definition, so should it return NULL
rather than false?

I found the issue when I read spec of MULTISET. In the SQL
standard, the following expression returns NULL. I was thinking
to make SUBMULTISET OF to be an alias of <@ operator, but
they seems to be incompatible.

- MULTISET[2] SUBMULTISET OF MULTISET[1, NULL] => NULL

Will we change the operator's behavior? It would be more
consistent, but incompatible with older server versions.
If impossible, I'll add separated submultiset_of() function
instead of <@ operator for MULTISET supports.

--
Itagaki Takahiro