class PQ{
   fcn init(numLevels=10){  // 0..numLevels, bigger # == lower priorty
      var [const] queue=(1).pump(numLevels+1,List.createLong(numLevels).write,L().copy);
   }
   fcn add(item,priorty){ queue[priorty].append(item); }
   fcn peek{ if(q:=queue.filter1()) return(q[-1]);   Void }// -->Void if empty
   fcn pop { if(q:=queue.filter1()) return(q.pop()); Void }// -->Void if empty
   var [private] state=L();
   fcn [private] next{ // iterate
      qi,ii:=state;
      foreach n in ([qi..queue.len()-1]){
         q:=queue[n];
	 if(ii>=q.len()) ii=0;
	 else{ state.clear().append(n,ii+1); return(q[ii]) }
      }
      Void.Stop
   }
   fcn walker{ state.clear().append(0,0); Walker(next) } // iterator front end
   fcn toString{ "PQ(%d) items".fmt(queue.reduce(fcn(sum,q){ sum+q.len() },0)) }
}