code: def _process_mod(self, key, down):
          
          if key in MOD_KEYS:
              if down:
                  if key not in self._mod:
                      self._mod.append(key)
              elif key in self._mod:
                  self._mod.pop(self._mod.index(key))
          return self._mod
docs: Process (possible) keyboard modifiers
  
          GLFW provides "mod" with many callbacks, but not (critically) the
          scroll callback, so we keep track on our own here.