JG connects certain prepositions ("of" and "for") to proper-noun
objects.              
		       +-MG--+--JG-+
                       |     |     |
	The National Academy of Science is meeting

Proper nouns have an optional "MG+"; certain prepositions like
"of" and "for" have "MG- & JG+". Since MG attaches a modifying
preposition to a noun, is analagous to M; and JG, which
attaches a preposition to its object, is analagous to J. But
note that MG+ on prepositions is conjoined only with JG+, not
with J+. Thus proper nouns can only be modified with
prepositional phrases using other proper nouns: "The National
Academy of Science is meeting", "*The National Academy of
science is meeting".  (Similarly, proper nouns taking JG can
only modify other proper nouns; but since they also have J-
connectors, they can use these to modify ordinary nouns: "My
book on Germany is excellent".)