S_non_monadic contains the core operations of a zipper, without any parametrisation over a particular failure monad.

S_monadic contains the core operations of a zipper, parametrised over a particular failure monad.

S contains S_non_monadic; a functor for generating S_monadic over a custom monad; and specialisations of it over common monads.

The type of instructions returned by functions used with fold_until_m and fold_until.

S_marked_non_monadic extends S_non_monadic to add functions for manipulating marks.

S_marked_monadic extends S_monadic to add functions for manipulating marks.

S_marked extends S to add functions for manipulating marks.

Basic_mark is the interface that mark types must implement.