ensure coerce only applies to multiplication

@fotos "(2 meters) * (3 meters)" is "6 square-meters". You don't have "square-USD", så that doesn't make sense. However, "(2) * (3 meters)" is "6 meters".

If you add in coercion, it should be (1) consistent, (2) intuitive. Otherwise, we should stick to what we learned in an elementary school physical class :-)