Indicates whether the line should be drawn before or after the track with
its index. This is mostly only relevant when gutter is used, since, then,
the position after a track is not the same as before the next
non-gutter track.
A possibly repeatable grid object.
It still exists even when not repeatable, but must not have additional
considerations by grid layout, other than for consistency (such as making
a certain group of rows unbreakable).