message: Prevent accidental re-use of removed field.
diff: @@ -30,7 +30,8 @@ message FunctionDef {
  // Attributes specific to this function definition.
  map<string, AttrValue> attr = 5;
  - // NOTE: field id 2 deleted on Jan 11, 2016, GraphDef version 21.
  + // NOTE: field id 2 deleted on Jan 11, 2017, GraphDef version 21.
  + reserved 2;
  // In both of the following fields, there is the need to specify an
  // output that is used as either the input to another node (in
  