Subset dispatch

[andrewallenbruce]

I think [ only dispatches on the first argument, so you need method(`[`, Klass) <- ...

The error Class specification must be an S7 class object, the result of `new_S3_class()`, an S4 class object, or a base class, not a <list> hints at this obliquely, since single-dispatch methods just take the class, not a list of classes.

Originally posted by @mjskay in RConsortium/S7#517 (comment)