# Global overlapped class-type hierarchy.
	# The hierarchy when all modules are combined.
	# Therefore, this hierarchy is used for a fast negative subtype check.
	#
	# This poset will evolve in an anarchic way. Loops can even be created.
	#
	# FIXME decide what to do on loops
	private var full_mtype_specialization_hierarchy = new POSet[MClassType]