Clarification of behavior for .isa() on built-in types

[Stevan Little]

Hello All,
So I am expanding our .isa() tests for built-in datatypes (in 
particular Array), and I have a few (probably very simple) questions 
(since I cannot seem to find details on this anywhere).

Is there an isa() built-in for this?
Or it is really @array.meta.isa() (from the Introspection section of 
S12)?

According to S12/Introspection, isa() does 2 things:
(NOTE: this my interpretation since the details are not speced)
- If given no arguments (only an invocant), it returns a list of the 
invocant's parent classes
- If given a class name as an argument, it returns a boolean (true if 
invocant.isa(class) and false otherwise)

Now some questions about 'class name' argument described in the second 
item above. (NOTE: I am asking general .isa() questions now, not just 
about built-ins).

I assume it can be a Class name: $fido.isa(Dog)
Or it can be a String:           $fido.isa('Dog')
But can it also be a Junction? :
        $fido.isa(Dog | Cat)    # true if $fido.isa(Dog) or $fido.isa(Cat)
        $fido.isa(Dog & Beagle) # true if $fide.isa(Dog) and $fido.isa(Beagle)
If it can be a Junction, it makes me wonder if maybe what is happening 
behind the scenes is not some variant of the "smart-match" (~~). In 
that case, then would something like this DWIM (ignoring the 
questionable sanity of actually doing it for a momemnt)

     @array.isa(@array)
or even:
        @array.isa([]) # basically @array.isa(List)
However, maybe this is going to far :)
Any clarification would be much appreciated.
Stevan