anchor.diff:  
   : app-loop
     begin
  -    heartbeat
  +    led-blinker
       button-check
     key? until ;
   
  
positive.code: 0 variable btn.timer  \ passed to periodic-ms
  0 variable led.timer  \ passed to periodic-ms
  
  : heartbeat [: LED3 iox! ;] led.timer 500 periodic-ms ;
  
  : btn-check-one ( pin bit -- f )  \ true if the button was just pressed
    bit swap ( mask pin )
  
negative.code: PA8  constant BTN-COMMON
  
  \ up to four status LEDs
  PB11 constant LED3  \ on = "1", off = "0" - heartbeat blinker
  PB10 constant LED4  \ on = "1", off = "0"
  PB1  constant LED5  \ on = "1", off = "0"
  PB0  constant LED6  \ on = "1", off = "0"
  