anchor.diff:        xbase <= cfg.xbase;
         xstep <= cfg.xstep;
         xlimit <= cfg.xlimit;
  +      xdown <= cfg.xlimit - cfg.xbase;
   
         first <= True;
         last <= (cfg.xbase+cfg.xstep >= cfg.xlimit);
  
positive.code: 	 let next_x = x + xstep;
  	 countReg <= countReg + 1;
  	 x <= x + xstep;
  	 first <= False;
  	 last <= (next_x+xstep >= xlimit);
  	 idle <= last;
        endmethod
        method Bool notEmpty();
  
negative.code:    Reg#(a) xstep <- mkReg(0);
     // inclusive limit
     Reg#(a) xlimit <- mkReg(0);
     Reg#(a) xdown <- mkReg(0);
     Reg#(Bool) first <- mkReg(False);
     Reg#(Bool) last <- mkReg(False);
     Reg#(Bool) idle <- mkReg(True);
  