protected override void OnInsert (Instruction item, int index)
{
if (size == 0)
return;
var current = items [index];
if (current == null) {
var last = items [index - 1];
last.next = item;
item.previous = last;
return;
}
var previous = current.previous;
if (previous != null) {
previous.next = item;
item.previous = previous;
}
current.previous = item;
item.next = current;
}