type elt
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