docs: public void step() {
          if (this.isSolved()) return;
          explain = true;
          session.setGlobal("explain", explain);
          this.counter.setCount(1);
          session.update(session.getFactHandle(this.counter), this.counter);
          if( steppingFactHandle == null ){
              steppingFactHandle = session.insert( stepping = new Stepping() );
          }
          this.session.fireUntilHalt();
          if( stepping.isEmergency() ){
              this.unsolvable = true;
          }
  //        dumpGrid();
      }