public void link(particle a, particle b){
if (a == null)
return;
if (b == null)
return;
a.link (b);
pair_st s = new pair_st (a, b);
if (!pDict.ContainsKey (s)) {
pDict.Add (s, new List<pair_part> ());
}
var par = new pair_part(a.id,b.id);
pDict [s].Add (par);
}
