A compression virus is an example of a benevolent computer virus, invented by Fred Cohen. It searches for an uninfected executable file, compresses the file and prepends itself to it. The virus can be described in pseudo code program compression-virus:={01234567;subroutine infect-executable:= {loop:file = get-random-executable-file; if first-line-of-file = 01234567 then goto loop; compress file; prepend compression-virus to file; }main-program:= {if ask-permission then infect-executable; uncompress the-rest-of-this-file into tmpfile; run tmpfile;}}