private static void CheckMatchError(StringMatcher m, Func <bool> fail)
{
int idx = m.StartIndex;
int len = m.Length;
fail().Should().BeFalse();
m.IsError.Should().BeTrue();
m.ErrorMessage.Should().NotBeNullOrEmpty();
m.StartIndex.Should().Be(idx, "Head must not move on error.");
m.Length.Should().Be(len, "Length must not change on error.");
m.ClearError();
}