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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def combine_graph_defs(to_proto, from_proto):
 """Combines two GraphDefs by adding nodes from from_proto into to_proto. All GraphDefs are expected to be of TensorBoard's. It assumes node names are unique across GraphDefs if contents differ. The names can be the same if the NodeDef content are exactly the same. Args: to_proto: A destination TensorBoard GraphDef. from_proto: A TensorBoard GraphDef to copy contents from. Returns: to_proto Raises: ValueError in case any assumption about GraphDef is violated: A GraphDef should have unique node, function, and gradient function names. Also, when merging GraphDefs, they should have not have nodes, functions, or gradient function mappings that share the name but details do not match. """

  if from_proto.version != to_proto.version:
    raise ValueError('Cannot combine GraphDefs of different versions.')

  try:
    _safe_copy_proto_list_values(
        to_proto.node,
        from_proto.node,
        lambda n: n.name)
  except _ProtoListDuplicateKeyError as exc:
    raise ValueError('A GraphDef contains non-unique node names: %s' % exc)
  except _SameKeyDiffContentError as exc:
    raise ValueError(
      ('Cannot combine GraphDefs because nodes share a name '
       'but contents are different: %s') % exc)
  try:
    _safe_copy_proto_list_values(
        to_proto.library.function,
        from_proto.library.function,
        lambda n: n.signature.name)
  except _ProtoListDuplicateKeyError as exc:
    raise ValueError('A GraphDef contains non-unique function names: %s' % exc)
  except _SameKeyDiffContentError as exc:
    raise ValueError(
      ('Cannot combine GraphDefs because functions share a name '
       'but are different: %s') % exc)

  try:
    _safe_copy_proto_list_values(
        to_proto.library.gradient,
        from_proto.library.gradient,
        lambda g: g.gradient_func)
  except _ProtoListDuplicateKeyError as exc:
    raise ValueError(
      'A GraphDef contains non-unique gradient function names: %s' % exc)
  except _SameKeyDiffContentError as exc:
    raise ValueError(
      ('Cannot combine GraphDefs because gradients share a gradient_func name '
       'but map to different functions: %s') % exc)

  return to_proto