output ports of choose, try

The spec says that the output ports of the children of choose (when,
otherwise), and try (group, catch), must match.  But I don't see
anywhere where it says that the choose or try itself has the
corresponding outputs (and that one of them is primary, if the
children have primary outputs).  This needs to be said so that
the sblings of the choose or try have those outputs in their
readable ports.

-- Richard