def missing_edges(x,y):
	"""
	Counts edges that are present in the original structure (x) but
	are missing in the learned structure (y). (lower is better)
	"""
	missing = 0
	for rv in x.nodes():
		if not y.has_node(rv):
			missing += x.degree(rv)
		else:
			for child in x.children(rv):
				if child not in y.children(rv):
					missing += 1
	return missing