let Point3 = (x, y, z) => {
    let parent = Point(x, y);
    let self = (message, args=null) => {
      switch (message) {
        // getters
        case 'z': return z

        // some operations
        // (immutable, but that's not required)

        case 'toString':
          return `Point(x=${x}, y=${y}, z=${z})`;

        case 'move':
          let [dx, dy, dz] = args;
          // use our getters
          return Point(self('x')+dx, self('y')+dy, self('z')+dz);

        // let's get DRY!
        case 'plus':
          let [other] = args;
          return self('move', other('x'), other('y'), other('z'));

        default:
          parent(msg, args)
      }
    };
    return self;
  };