code: protected final Interaction findInteraction(String id) {
          for(Node n= interactions.head(), end= interactions.tail(); (n= n.next()) != end;) {
              Interaction i= (Interaction)n.value();
              
              if(i.id.equals(id)) {
                  return i;
              }
          }
          
          return null;
      }
docs: Caller must hold the lock on interactions!