anchor.diff:  \ Dr. C. H. Ting and Juergen Pintaske, as of 2013-Apr-20.
   \ See http://www.exemark.com/FORTH/eForthOverviewv5.pdf
   
  +
  +\ ?next decrements the top of return stack.  If it falls
  +\ BELOW zero (e.g., goes negative), then the branch back
  +\ to a corresponding FOR-loop's entry is NOT taken.
  +tcode ?next
  +	0 rsp x9 ld,
  +	-1 x9 x9 addi,
  +	16 x9 x0 blt,
  +	0 rsp x9 sd,
  +	0 ip ip ld,
  +	next,
  +	8 rsp rsp addi,
  +	8 ip ip addi,
  +	next,
  +tend-code
  +
  +tcode (dofor)
  +	0 dsp x9 ld,
  +	8 dsp dsp addi,
  +	-1 x9 x9 addi,
  +	-8 rsp rsp addi,
  +	0 rsp x9 sd,
  +	next,
  +tend-code
   
   \ ?branch causes a control flow change if the TOS is zero.
   \ This primitive consumes the stack item.
  
positive.code: \ TODO test
  : ~AGAIN	[t'] branch t, t, ;
  : ~UNTIL	[t'] ?branch t, t, ;
  
  \ PANIC will stop all program execution until the machine
  \ is physically reset.  This includes interrupt handlers.
  
negative.code: 	0 dsp x8 sd,
  	next,
  tend-code
  
  \ SP@ and SP! retrieve and reset the data stack pointer.
  \ BE EXTREMELY CAREFUL WHEN USING SP!, AS THIS CAN LEAD
  