 directions.
    compatiblePorts(p1, p2) {
      // already connected?
      const part1 = p1.part;
      const id1 = p1.portId;
      if (id1 === null || id1 === '') return false;
      if (part1.findLinksConnected(id1).filter((l) => l.category === '').count > 0) return false;
      const part2 = p2.part;
      const id2 = p2.portId;
      if (id2 === null || id2 === '') return false;
      if (part2.findLinksConnected(id2).filter((l) => l.category === '').count > 0) return false;
      // compatible fitti