val zip : 'a list -> 'b list -> ('a * 'b) listval unzip : ('a * 'b) list -> 'a list * 'b listval zip_with : ('a -> 'b -> 'c) -> 'a list -> 'b list -> 'c listval map_opt : ('a -> 'b option) -> 'a list -> 'b list optionval map_accum_left : ('a -> 'b -> 'a * 'c) -> 'a -> 'b list -> 'a * 'c list