if (this.next.p1 != p2) throw new Error();
if (this.prev.p2 != p1) throw new Error();
if (this.p1.e == null) this.p1.e = this;
if (this.next.p1 != p2) throw new Error();
if (this.prev.p2 != p1) throw new Error();
if (this.p1.e == null) this.p1.e = this;