old_contents: """Utilities related to the IncidentNodeTag values."""
  
  from tensorflow_gnn.graph import graph_constants as const
  
  
  def reverse_tag(tag):
    """Flips tfgnn.SOURCE to tfgnn.TARGET and vice versa."""
    if tag == const.TARGET:
      return const.SOURCE
    elif tag == const.SOURCE:
      return const.TARGET
    else:
      raise ValueError(
          f"Expected tag tfgnn.SOURCE ({const.SOURCE}) "
          f"or tfgnn.TARGET ({const.SOURCE}), got: {tag}")
  
new_contents: """Utilities related to the IncidentNodeTag values."""
  
  from tensorflow_gnn.graph import graph_constants as const
  
  
  def reverse_tag(tag):
    """Flips tfgnn.SOURCE to tfgnn.TARGET and vice versa."""
    if tag == const.TARGET:
      return const.SOURCE
    elif tag == const.SOURCE:
      return const.TARGET
    else:
      raise ValueError(
          f"Expected tag tfgnn.SOURCE ({const.SOURCE}) "
          f"or tfgnn.TARGET ({const.TARGET}), got: {tag}")
  
subject: Fix to exception message.