func (l Line) Duration() time.Duration {
d := time.NoDuration()
for _, e := range l.elements {
d = d.Add(e.Duration())
}
return d
}
func (l Stack) Duration() time.Duration {
d := time.NoDuration()
for _, v := range l.elements {
if v.Duration().Greater(d) {
d = v.Duration()
}
}
return d
}