anchor.diff:  element -> message_def:                 '$1'.
   %% element -> extend_def:                  '$1'.
   element -> option_def:                  '$1'.
  +%% element -> service_def                  '$1'.
   
   package_def -> package name ';':        {package, '$2'}.
   
  
positive.code:         '$end'.
  
  
  proto -> elements:                      '$1'.
  
  elements -> element elements:           ['$1' | '$2'].
  elements -> ';' elements:               '$2'.
  
negative.code:         default
          import
          option
          %% '(' and ')' for custom options
          %% 'extensions', 'extend', 'max' and 'to' for extensions
          %% 'service', 'rpc', 'returns', '(' and ')' for services
          packed deprecated
          '.' ';' '{' '}' '[' ']' '=' ','
          .
  