anchor.diff:  // Fields
   //
   
  -type
  +typeRule
       :   (   'double'
           |   'float'
           |   'int32'
  
positive.code: // Normal field
  
  field
      :   'repeated'? type fieldName '=' fieldNumber ('[' fieldOptions ']')? ';'
      ;
  
  fieldOptions
  
negative.code:     ;
  
  oneofField
      :   typeRule fieldName '=' fieldNumber ('[' fieldOptions ']')? ';'
      ;
  
  // Map field
  
  mapField
      :   'map' '<' keyType ',' typeRule '>' mapName '=' fieldNumber ('[' fieldOptions ']')? ';'
      ;
  
  keyType
  