public static void ContainsTest()
{
var r = Relations.Make <string, int>((s, i) => s.Length == i);
Assert.True(r.Contains("", 0));
Assert.True(r.Contains("x", 1));
Assert.True(r.Contains("xyz", 3));
Assert.False(r.Contains("", 1));
Assert.False(r.Contains("x", 0));
Assert.False(r.Contains("x", -1));
Assert.False(r.Contains("xyz", 88));
}