code: def find_slot(cls, ctx):
          
          Dr7  = ctx['Dr7']
          slot = 0
          for m in cls.enableMask:
              if (Dr7 & m) == 0:
                  return slot
              slot += 1
          return None
docs: Finds an empty slot to set a hardware breakpoint.
  
          @see: clear_bp, set_bp
  
          @type  ctx: dict( str S{->} int )
          @param ctx: Thread context dictionary.
  
          @rtype:  int
          @return: Slot (debug register) for hardware breakpoint.