Accessor.Indexedtype ('inner_index, 'outer_index, 'inner, 'outer, 'kind) t =
('inner_index -> 'inner -> 'inner, 'outer_index -> 'outer -> 'outer, 'kind)
General.tAccessors commonly don't need to support polymorphic updates, making General.t excessively verbose, but t is not useable for indexed accessors. In such cases, you can use Indexed.t to reduce some boilerplate.