 On the second, Q is so extensive or so resistant to intelligible compact expression for other reasons that the benefits to speaker and listener alike of a terse and simple utterance outweigh the inferential costs to the listener involved in extracting a large message from a small package. This is often the case with allusions, for instance. Where outright explicit assertion can’t be managed or can be managed only with excessive effort on both sides, suggestion has to suffice and will violate no general conversational norms.