rooted DAG != tree, so please don't introduce people to a concept by explicitly misleading them.
All nodes in a tree must have exactly 1 parent, except for the root that has exactly 0 parents. All nodes in a (single-)rooted DAG must have at least one parent, except for the root that has exactly 0 parents.
What I said was wrong. It was also doomed to be sloppy at best, because I intended to compare trees as programmers know them (i.e., typically directed and rooted, which is more structure than mathematicians' 'trees' have) to DAGs, which have a precise mathematical definition.
But I hardly meant to introduce anyone to the concept of either DAGs or trees by that comment. If there's a chance of it seriously misleading anyone, I would be happy to see it moderated out of sight.
The point I intended to make, more carefully stated, is this:
DAGs are closely related to structures that programmers already deal with all the time, and so it strikes me as odd when programmers protest learning to work with DAGs.
All nodes in a tree must have exactly 1 parent, except for the root that has exactly 0 parents. All nodes in a (single-)rooted DAG must have at least one parent, except for the root that has exactly 0 parents.