code: def merge_prefix(prefix, key):
      if prefix:
          return "/".join((prefix, key))
      else:
          return key
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import types
  import tensorflow.compat.v1
label_desc: Use image node_id 3 for calling a global function with example usage: merge_prefix(prefix, key) and returns: str, key