private void TestAllLengths(MemoryPoolBlock2 block, int lengths)
 {
     for (var firstIndex = 0; firstIndex <= lengths; ++firstIndex)
     {
         for (var lastIndex = firstIndex; lastIndex <= lengths; ++lastIndex)
         {
             var first = block.GetIterator().Add(firstIndex);
             var last = block.GetIterator().Add(lastIndex);
             Assert.Equal(firstIndex, block.GetIterator().GetLength(first));
             Assert.Equal(lastIndex, block.GetIterator().GetLength(last));
             Assert.Equal(lastIndex - firstIndex, first.GetLength(last));
         }
     }
 }