code: def _add_hotkey_step(handler, combinations, suppress):
      
      container = _listener.blocking_hotkeys if suppress else _listener.nonblocking_hotkeys
  
      # Register the scan codes of every possible combination of
      # modfiier + main key. Modifiers have to be registered in 
      # filtered_modifiers too, so suppression and replaying can work.
      for scan_codes in combinations:
          for scan_code in scan_codes:
              if is_modifier(scan_code):
                  _listener.filtered_modifiers[scan_code] += 1
          container[scan_codes].append(handler)
  
      def remove():
          for scan_codes in combinations:
              for scan_code in scan_codes:
                  if is_modifier(scan_code):
                      _listener.filtered_modifiers[scan_code] -= 1
              container[scan_codes].remove(handler)
      return remove
docs: Hooks a single-step hotkey (e.g. 'shift+a').