code: public void delete(Vertex vtx) {
          if (vtx.prev == null) {
              head = vtx.next;
          } else {
              vtx.prev.next = vtx.next;
          }
          if (vtx.next == null) {
              tail = vtx.prev;
          } else {
              vtx.next.prev = vtx.prev;
          }
      }
docs: Deletes a vertex from this list.