Empty_hashes (empty_hashes.Empty

module type Hash_intf = sig ... end

val merge_hash : 
  (module Hash_intf with type t = 'hash) ->
  int ->
  'hash0 ->
  'hash1

val cache : 
  (module Hash_intf with type t = 'a) ->
  init_hash:'b ->
  int ->
  'c Immutable_array.t

val extensible_cache : 
  (module Hash_intf with type t = 'a) ->
  init_hash:'b ->
  int ->
  'c