fcn recamanW{  // -->iterator -->(n,a,True if a is a dup)
   Walker.tweak(fcn(rn,rp,d){
      n,p,a := rn.value, rp.value, p - n;
      if(a<=0 or d.find(a)) a+=2*n;
      d.incV(a); rp.set(a);
      return(rn.inc(),a,d[a]>1);
   }.fp(Ref(0),Ref(0),Dictionary()) )
}