private static IEnumerable <Gap> findGaps(ISortedEnumerable <int> filledPositionIndices, Interval toFill)
{
int previousPresentIndex = toFill.Start - 1;
foreach (int presentIndex in filledPositionIndices.ConcatIfNotLastElement(toFill.End))
{
if (presentIndex != previousPresentIndex + 1)
{
yield return(new Gap(previousPresentIndex + 1, presentIndex));
}
previousPresentIndex = presentIndex;
}
}
