Corresponds to the "version number" concept in many host file systems.
The version is either a positive integer
or a symbol from the following list:
nil, :wild
, :unspecific
, or :newest
(refers to the largest version number that already exists in
the file system when reading a file, or to
a version number
greater than any already existing in the file system
when writing a new file). Implementations
can define other special version symbols.