anchor.diff:  Nonterminals
           proto
           elements element
  +        enum_def enum_fields enum_field
  +        opt_enum_opts enum_opts enum_opt
           message_def msg_elems msg_elem
  -        enum_def enum_fields enum_field
  -        field_opts field_opt cardinality type
  +        opt_field_opts field_opts field_opt cardinality type
           package_def
           import_def
           identifiers
  
positive.code:               reformat_names(flatten_defs(absolutify_names(Defs)))))).
  
  
  fetches_imports_test() ->
      {ok, Elems} = parse_lines(["package p1;"
                                 "import \"a/b/c.proto\";",
  
negative.code:                                                name=identifier_name('$3'),
                                                 fnum=literal_value('$5'),
                                                 opts=[]}.
  msg_elem -> cardinality type identifier '=' dec_lit '[' opt_field_opts ']' ';':
                                          #field{occurrence='$1',
                                                 type='$2',
                                                 name=identifier_name('$3'),
  