// later: transitive closure over mapped_follows()
.decl later(later_l:label, earlier_l:label)
later(l2, l1) :- mapped_follows(l2, l1).
later(l3, l1) :- later(l3, l2), later(l2, l1).

// earlier: this is a convenience wrapper
.decl earlier(earlier_l:label, later_l:label)
earlier(l1, l2) :- later(l2, l1).

.decl equal_or_earlier(earlier_l:label, later_l:label)
equal_or_earlier(l, l) :- mapped_follows(l, _).
equal_or_earlier(l, l) :- mapped_follows(_, l).
equal_or_earlier(l1, l2) :- earlier(l1, l2).

.decl first(l:label)
first(l) :- mapped_follows(_, l), !later(l, _).

.decl last(l:label)
last(l) :- mapped_follows(l, _), !earlier(l, _).