The version can be any symbol or any integer.
The symbol :newest refers to the largest version number 
that already exists in the file system
when reading, overwriting, appending, superseding, or directory listing 
an existing file.
The symbol :newest refers to the smallest version number
greater than any existing version number when creating a new file.
The symbols nil, :unspecific, and :wild have special meanings and
restrictions; see Special Pathname Component Values and Restrictions on Constructing Pathnames.
Other symbols and integers have implementation-defined meaning.