> I'd propose "in" infix operator.
...
> What do you think?

I like it.  There's a kind of elegance in it

if a in [1,2,3]
    p ' it is in 1,2,3'
end

It works well with my head.  +1

The object oriented way would also be acceptable to me, I think, so
one or the other is fine.

if a.in? [1,2,3]
    p ' it is in 1,2,3'
end

=== would work except not for arrays:

>> 11 === [11,22,33]
=> false

So I guess it would need to handle that case somehow.

Cheers.
-r