U is a special link used with nouns. In most cases,
nouns have both a determiner requirement ("D-") and a main
subject-object requirement ("S+ or O- or J-").  In a few
cases, however, a single word appears to satisfy both
requirements:

               +--U---+
               |      |
	What kind_of dog did you buy	

"Dog" would seem to be acting like the prepositional object of
"of" here; but, unusually, it requires no determiner.
Therefore we give nouns a "U-" connector, disjoined with both
its "D-" and its "S+ or O- or J-..." complex.

Other uses include the following:

		               +--U--+
                               |     |
	We spend four dollars per student
	We spend four dollars  a  student

For the latter case, we give the word "a" the expression
"[[Mp- or Us+]]". Since this usage is rare compared to
the other uses of "a", we give it a cost of 2.

U#c is used in comparatives; see "MV: Comparatives IV".