anchor.diff:  const int kNumFireworkParticles = 200;
   const double kMaxDistance = 100.0;
   const double kMinDistance = 0.2;
  -const int kPoles = 10;
  -const int kIons = 100000;
  +const int kIons = 10000;
   
   String DumpVec(VM.Vector3 v) {
     return "${v.x} ${v.y} ${v.z}";
  
positive.code:       // stop if we are out of bounds
        if (len > kMaxDistance) {
          Pole p = srcs[rng.nextInt(srcs.length)];
          _pos.xyz = p.Pos().xyz;
          print("too far ${len}");
          return;
        }
  
negative.code: }
  
  class Ion {
    VM.Vector3 _pos;
    double _speed;
  
    Ion(this._speed, VM.Vector3 pos) {
      _pos = pos.clone();
    }
  
    VM.Vector3 Pos() => _pos;
  