code: def join(a, b, sep="."):
      """
      Joins `a` and `b` using `sep`.
      """
      if not a:
          return b
  
      return f"{a}{sep}{b}"
  
file_path: gluonts/src/gluonts/core/serde/flat.py
imports: from collections import defaultdict
  from itertools import count
  from typing import Any
  from toolz.dicttoolz import keymap, valmap
  from ._base import encode, Kind, decode
label_desc: Use image node_id 1 for calling a global function with example usage: join(a, b, sep) and returns: a, sep, b, b