docs: def config(check):
      """Validate default configuration files."""
      if check:
          checks = [check]
      else:
          checks = sorted(get_valid_checks())
  
      files_failed = {}
      files_warned = {}
      num_files = 0
  
      echo_waiting('Validating default configuration files...')
      for check in checks:
          check_display_queue = []
  
          config_files = get_config_files(check)
          for config_file in config_files:
              num_files += 1
              file_display_queue = []
              file_name = basepath(config_file)
  
              try:
                  config_data = yaml.safe_load(read_file(config_file))
              except Exception as e:
                  files_failed[config_file] = True
  
                  # We must convert to text here to free Exception object before it goes out of scope
                  error = str(e)
  
                  check_display_queue.append(lambda: echo_info('{}:'.format(file_name), indent=True))
                  check_display_queue.append(lambda: echo_failure('Invalid YAML -', indent=FILE_INDENT))
                  check_display_queue.append(lambda: echo_info(error, indent=FILE_INDENT * 2))
                  continue
  
              # Verify there is an `instances` section
              if 'instances' not in config_data:
                  files_failed[config_file] = True
                  file_display_queue.append(lambda: echo_failure('Missing `instances` section', indent=FILE_INDENT))
  
              # Verify there is a default instance
              else:
                  instances = config_data['instances']
                  if check not in IGNORE_DEFAULT_INSTANCE and not isinstance(instances, list):
                      files_failed[config_file] = True
                      file_display_queue.append(lambda: echo_failure('No default instance', indent=FILE_INDENT))
  
              if file_display_queue:
                  check_display_queue.append(lambda: echo_info('{}:'.format(file_name), indent=True))
                  check_display_queue.extend(file_display_queue)
  
          if check_display_queue:
              echo_success('{}:'.format(check))
              for display in check_display_queue:
                  display()
  
      files_failed = len(files_failed)
      files_warned = len(files_warned)
      files_passed = num_files - (files_failed + files_warned)
  
      if files_failed or files_warned:
          click.echo()
  
      if files_failed:
          echo_failure('Files with errors: {}'.format(files_failed))
  
      if files_warned:
          echo_warning('Files with warnings: {}'.format(files_warned))
  
      if files_passed:
          if files_failed or files_warned:
              echo_success('Files valid: {}'.format(files_passed))
          else:
              echo_success('All {} configuration files are valid!'.format(num_files))
  
      if files_failed:
          abort()