EqualityIdentity.fs

let createComparer (map) = 
    {
        new EqualityComparer<_>() with
        override this.Equals(a, b) = Unchecked.equals (map a) (map b)
        override this.GetHashCode(a) = Unchecked.hash (map a)        
    }

module Seq = 
    let exceptWith map first second  =
        Enumerable.Except(first, second, createComparer map)

(*

Example:
> [(1, 2); (3, 4)] |> Seq.exceptWith fst [(1, 3); (5, 7)];;
val it : IEnumerable<int * int> = seq [(5, 7)]

*)