docs: def add(ctx, alias, mapping, backend):
      """
      Add a new alias to your configuration file.
      """
      if not backend:
          backends_list = ctx.obj['settings'].get_backends()
          if len(backends_list) > 1:
              raise click.UsageError(
                  "You're using more than 1 backend. Please set the backend to "
                  "add the alias to with the --backend option (choices are %s)" %
                  ", ".join(dict(backends_list).keys())
              )
  
      add_mapping(ctx, alias, mapping, backend)