I can’t help but wonder.
if (a !in b)
reads “if a is not in b”
if (a !is B)
reads "if a not is B"
which is a bit awkward, “if a is not B” would be more natural which basically translates to
if (a is! B)
Was this a conscious choice? Has this some more nuanced rationale?
is! are equally easy. They are are just some kind of special token similar to other keywords. The syntax itself does not get ambigous.
It is a kind of inconsistency because it’s
is! but to me the natural language flow would more than offset the inconsistency.