docs: public void solve() {
          if (this.isSolved()) return;
          explain = false;
          session.setGlobal("explain", explain);
          if( steppingFactHandle != null ){
              session.delete( steppingFactHandle );
              steppingFactHandle = null;
              stepping = null;
          }
          this.session.fireAllRules();
  //        dumpGrid();
      }