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