private static Occurrences Subtract(Occurrences x, Occurrences y)
{
var xm = x.ToDictionary(tuple => tuple.Item1, tuple => tuple.Item2);
var ym = y.ToDictionary(tuple => tuple.Item1, tuple => tuple.Item2);
var zm = new Dictionary <char, int>();
foreach (var kvp in xm)
{
var c = kvp.Key;
var nx = kvp.Value;
int ny;
zm[c] = (ym.TryGetValue(c, out ny)) ? nx - ny : nx;
}
return(zm
.Where(kvp => kvp.Value > 0)
.Select(kvp => Tuple.Create(kvp.Key, kvp.Value))
.OrderBy(tuple => tuple.Item1));
}
