5 val find: 'k -> ('k,'a) map -> 'a
7 val add: 'k -> 'a -> ('k,'a) map -> ('k,'a) map
9 val remove: 'k -> ('k,'a) map -> ('k,'a) map
11 val restrict: ('k -> bool) -> ('k,'a) map -> ('k,'a) map
13 val fold: ('k -> 'a -> 'b -> 'b) -> ('k,'a) map -> 'b -> 'b