fcn yellowstoneW{	// --> iterator
   Walker.zero().tweak(fcn(a,b){
      foreach i in ([1..]){
         if(not b.holds(i) and i.gcd(a[-1])==1 and i.gcd(a[-2]) >1){
	    a.del(0).append(i);	// only keep last two terms
	    b[i]=True;
	    return(i);
	 }
      }
   }.fp(List(2,3), Dictionary(1,True, 2,True, 3,True))).push(1,2,3);
}