Commands can be specified using the Term.eval_choice function, however there doesn't seem to be any way to specify sub commands, to achieve results such as opam pin add, where add is a sub command of pin.

More generally, it would be useful to be able to define arbitrary nesting of commands, possible through the use of a function with the same semantics as Term.eval_choice, but with the following signature:

val group : ('a Term.t * Term.info) list -> 'a Term.t