With the development of the Internet, illegal copying of electronic documents has become a growing problem. There is an increasing need of prevention in the field of pirate copying. One method is to mark the document by changing some of the words to synonyms. In this way it is possible to construct legal copies which do not differ in content but still are unique. Since the copies of the documents are unique, it is possible to trace the owner of a document and accordingly call him or her to account for pirate copying if several exactly similar copies are reaching the market.