type ('k,'a) map

val empty: ('k,'a) map

val find: 'k -> ('k,'a) map -> 'a

val add: 'k -> 'a -> ('k,'a) map -> ('k,'a) map

val remove: 'k -> ('k,'a) map -> ('k,'a) map

val restrict: ('k -> bool) -> ('k,'a) map -> ('k,'a) map

val fold: ('k -> 'a -> 'b -> 'b) -> ('k,'a) map -> 'b -> 'b