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Interpret a list of (key, value) pairs as a map in which only the first occurrence of a key affects the semantics, i.e.:

List.Assoc.xxx alist ...args... 

is always the same as (or at least sort of isomorphic to):

Map.xxx (alist |> Map.of_alist_multi |> Map.map ~f:List.hd) ...args... 
type ('a, 'b) t = ('a * 'b) list
include Sexpable.S2 with type ('a, 'b) t := ('a, 'b) t
val t_of_sexp : (Sexp.t -> 'a) -> (Sexp.t -> 'b) -> Sexp.t -> ('a, 'b) t
val sexp_of_t : ('a -> Sexp.t) -> ('b -> Sexp.t) -> ('a, 'b) t -> Sexp.t
val add : ('a, 'b) t -> equal:('a -> 'a -> bool) -> 'a -> 'b -> ('a, 'b) t
val find : ('a, 'b) t -> equal:('a -> 'a -> bool) -> 'a -> 'b option
val find_exn : ('a, 'b) t -> equal:('a -> 'a -> bool) -> 'a -> 'b
val mem : ('a, 'b) t -> equal:('a -> 'a -> bool) -> 'a -> bool
val remove : ('a, 'b) t -> equal:('a -> 'a -> bool) -> 'a -> ('a, 'b) t
val map : ('a, 'b) t -> f:('b -> 'c) -> ('a, 'c) t
val inverse : ('a, 'b) t -> ('b, 'a) t

Bijectivity is not guaranteed because we allow a key to appear more than once.