fcn [private] _permuteW(seq){	// lazy version
   N:=seq.len(); NM1:=N-1;
   ds:=(0).pump(N,List,T(Void,-1)).copy(); ds[0]=0; // direction to move e: -1,0,1
   es:=(0).pump(N,List).copy();  // enumerate seq

   while(1) {
      vm.yield(es.pump(List,seq.__sGet));

      // find biggest e with d!=0
      reg i=Void, c=-1;
      foreach n in (N){ if(ds[n] and es[n]>c) { c=es[n]; i=n; } }
      if(Void==i) return();

      d:=ds[i]; j:=i+d;
      es.swap(i,j); ds.swap(i,j);	// d tracks e
      if(j==NM1 or j==0 or es[j+d]>c) ds[j]=0;
      foreach e in (N){ if(es[e]>c) ds[e]=(i-e).sign }
   }
}

fcn permuteW(seq) { Utils.Generator(_permuteW,seq) }