code: public void delete(Vertex vtx1, Vertex vtx2) {
          if (vtx1.prev == null) {
              head = vtx2.next;
          } else {
              vtx1.prev.next = vtx2.next;
          }
          if (vtx2.next == null) {
              tail = vtx1.prev;
          } else {
              vtx2.next.prev = vtx1.prev;
          }
      }
docs: Deletes a chain of vertices from this list.