This is the main difference to actual behaviour from before, previously `TraversableOnce` was just mapped, but not only is `TraversableOnce.map` is a lazy operation, the result of it can only be traversed once even if the actual instance that's being mapped here is `Traversable`.  Now, after mapping it, we convert it to `immutable.Seq`, forcing it to be eagerly mapped, and thus guaranteeing that there are no mutable structures in the rendered template hierarchy.