I was trying to re-use some template XML in an external file. Normally, if this is within a snippet, you have to use eager_eval, correct? Otherwise the embed will not be evaluated, because it's outside-in, and the embed element was subject to some other CSS selector transformations (I neglected to say that in the previous post). The result was the embed didn't appear.
However, adding eager_eval to the comet parameters appeared to make no difference.
Dan