- type elt-  = t
- val equal : elt list -> elt list -> bool
- val mem : elt -> elt list -> bool
- val assoc : elt -> (elt * 'a) list -> 'a
- val mem_assoc : elt -> (elt * 'a) list -> bool
- val remove_assoc : elt -> (elt * 'a) list -> (elt * 'a) list
- val mem_assoc_sym : elt -> ('a * elt) list -> bool