# Hierarchy of class definition.
	#
	# Each classdef is associated with its super-classdefs in regard to
	# its module of definition.
	#
	# ~~~
	# var m = new ModelDiamond
	# assert     m.mclassdef_hierarchy.has_edge(m.mclassdef_b, m.mclassdef_a)
	# assert not m.mclassdef_hierarchy.has_edge(m.mclassdef_a, m.mclassdef_b)
	# assert not m.mclassdef_hierarchy.has_edge(m.mclassdef_b, m.mclassdef_c)
	# ~~~
	var mclassdef_hierarchy = new POSet[MClassDef]