removeSubList method
Implementation
VertexList removeSubList(VertexNode a, VertexNode b) {
if (a.prev == null) {
head = b.next;
} else {
a.prev!.next = b.next;
}
if (b.next == null) {
tail = a.prev;
} else {
b.next!.prev = a.prev;
}
return this;
}