code: def register_writer(klass):
      
      if not callable(klass):
          raise ValueError("Can only register callables as engines")
      engine_name = klass.engine
      _writers[engine_name] = klass
docs: Add engine to the excel writer registry.io.excel.
  
      You must use this method to integrate with ``to_excel``.
  
      Parameters
      ----------
      klass : ExcelWriter