/usr/lib/swipl/library/filesex.pl
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 relative_file_name(+Path:atom, +RelToFile:atom, -RelPath:atom) is det
relative_file_name(-Path:atom, +RelToFile:atom, +RelPath:atom) is det
True when RelPath is Path, relative to the file RelToFile. Path and RelTo are first handed to absolute_file_name/2, which makes the absolute and canonical. Below are two examples: 
?- relative_file_name('/home/janw/nice',
                      '/home/janw/deep/dir/file', Path).
Path = '../../nice'.

?- relative_file_name(Path, '/home/janw/deep/dir/file', '../../nice').
Path = '/home/janw/nice'.

Add a terminating / to get a path relative to a directory, e.g.

?- relative_file_name('/home/janw/deep/dir/file', './', Path).
Path = 'deep/dir/file'.
Arguments:
All- paths must be in canonical POSIX notation, i.e., using / to separate segments in the path. See prolog_to_os_filename/2.
bug
- It would probably have been cleaner to use a directory as second argument. We can not do such dynamically as this predicate is defined as a syntactical operation, which implies it may be used for non-existing paths and URLs.