val is_empty : 'a t -> boolval mem : env -> key -> 'a t -> boolval add : env -> key -> 'a -> 'a t -> 'a tval fold : (key -> 'a -> 'b -> 'b) -> 'a t -> 'b -> 'bval merge :
(key -> 'a option -> 'b option -> 'c option) ->
'a t ->
'b t ->
'c tval find : env -> key -> 'a t -> 'aval find_opt : env -> key -> 'a t -> 'a option