//Exam '13 assign

type Multiset<'a when 'a : equality> = ('a * int) list;;
//hvad fck er 'a og equality???

let abe = [("a",4);("b",5);("c",3);("o",9)]//Hvorfor kan det ikke overloades til Multiset??

let rec inv = function
    |[] -> true
    |m::ms' -> not(List.exists (fun x -> x=m) ms') && inv ms'

//insert: 'a -> int -> Multiset<'a> -> Multiset<'a>

let insert e n ms = function
    |((e,n),ms)
            when List.exists (fun x -> x<>(e,n)) ms -> [(e,n)]@ms
    |((e,n),ms)                                     -> List.foldBack
                                                          (fun (a,b) list ->if (a,b)<>(e,n)
                                                                            then (a,b)::list
                                                                            else (a,b+n)::list) ms []
//testcases, virker ikke?
let abe2 = insert "p" 5 abe
let abe3 = insert "p" 4 abe2