 A?
  bcs :+     ; no -> go on
  rts         ; return no letter
:
  cmp #$5b    ; smaller then Z+1?
  bcs :+     ; no -> go on
  sec         ; return letter
  rts
:  
  cmp #$61    ; smaller then a?
  bcs :+     ; no -> go on
  rts         ; return no letter
:  
  cmp #$7b    ; smaller then z+1?        
  bcs :+     ; no -> go on
  sec         ; return letter
  rts
:
  clc         ; return no letter
  rts        
        


; -------------------------------------
; test digit
;
; c