operator embellishment

Bruce writes:

> However, it might be necessary for one of the arguments to an embellisher
> (the operator or the e.g. subscript) to itself be, or contain,
> an embellished operator. This may be rarely desired, but it is allowed
> in our proposal, and could be expressed, for example, by
>         a {+_2}_3 b
> for
>         a +   b
>            2
>             3
> or by
>         a +_{b +_2 c} d
> for
>         a +       d
>            b +  c
>               2
> [which are admittedly extremely contrived examples].

Following is perhaps a more "natural" example, insofar as it
occurs in our literature and is in the first journal issue I
pulled from the shelves.  I'll use TeX coding:

Now the functor which sends an $R\otimes_S S_n$-module $M$ to
$R_{m,n}\otimes_{R\otimes_S S_n}M$ is the composition of ...

(So Bruce's example isn't "extremely contrived" at all --- it has the
same form as this "real" example.)