docs: def pass_key(f):
      """
      helper function for fmap of dict to concern only on values.
      `pass_key(f)` would return a function which shadow input `key` and combine it
      with return of `f(value)` to `(key, f(value))`.
      """
      @func(nargs=1, nouts=1)
      def wrapper(k, v):
          return (k, f(v))
      return wrapper