There exists a special Czech and Slovak typographical rule: you cannot leave the non-syllabic preposition on the end of one line and continue writting text on next line. For example, you cannot write down the text "v lese" (in a forest) like "v<new-line>lese". The program vlna adds the asciitilde between such preposition and the next word and removes the space(s) in this place. It means, the program converts "v lese" to "v~lese". You can use this program as a preporcessor before TeXing. Moreower, you can set another sequence to store instead asciitilte (see the -x option).