Make this stemmer use the same construct as the others:

    [substring] among (
        // cases
    )

rather than:

    [among ( (])
        // cases
    )

The second version seems more cryptic, and I suspect dates back to
before substring was added to the language.

Both result in almost identical code being generated - aside from
different comments, the only difference is that the new version avoids
an interim assignment to among_var in one case which is arguably better
though the compiler will probably optimise both versions to the same
machine code.