Integers are interpreted as revision numbers.
SRFI-19 time objects are interpreted as points in time specifying a revision.
The symbol 'unspecified indicates that some default value for the revision number should be chosen by the Subversion library.
The symbol 'committed indicates the revision of the most recent change.
The symbol 'previous indicates the revision of the most recent change minus one.
The symbol 'base indicates the current revision stored in ".svn/entries".
The symbol 'working indicates the uncommitted state of work in a working copy.
The symbol 'head indicates the most recent revision in the repository.