anchor.diff:  \ current top of the return stack.  R@ is also known as I,
   \ for applications working with DO...LOOP constructs.
   
  -tcode R>	\ TODO test
  +tcode R>
   	0 rsp x8 ld,
   	8 rsp rsp addi,
   	-8 dsp dsp addi,
  
positive.code: : ~BEGIN	there ;
  : ~WHILE	~IF swap ;
  : ~REPEAT	[t'] branch t, t, ~THEN ;
  : ~FOR		[t'] (dofor) t, ~BEGIN ;
  : ~NEXT		[t'] ?next t, t, ;
  : ~AHEAD	[t'] branch t, there 0 t, ;
  : ~AFT		drop ~AHEAD ~BEGIN swap ;
  
negative.code: 	next,
  tend-code
  
  tcode R@
  	0 rsp x8 ld,
  	-8 dsp dsp addi,
  	0 dsp x8 sd,
  