public static AndConstraint <StringCollectionAssertions> OnlyContain(
     this StringCollectionAssertions assertions, IReadOnlyCollection <string> expected, string because = "", params object[] reasonArgs)
 => assertions.HaveCount(expected.Count, because, reasonArgs).And.Contain(expected, because, reasonArgs);
 public static AndConstraint <StringCollectionAssertions> OnlyContain(
     this StringCollectionAssertions assertions, params string[] expected)
 => assertions.HaveCount(expected.Length).And.Contain(expected);