f = section/test.foo
 g = dir/subdir/
 abs = /tmp
 f.as_directory() = section/test.foo/
 f.as_file() = section/test.foo
 g.as_directory() = dir/subdir/
 g.as_file() = dir/subdir
 f is file
 g is directory
 g is relative
 abs is absolute
 g + f = dir/subdir/section/test.foo
 g + abs = /tmp
 abs + g + f = /tmp/dir/subdir/section/test.foo