void verify() throws MVDException
	{
		BitSet bs1=null,bs2=null;
		if ( incoming.size() > 0 )
			bs1 = checkArcs( incoming.listIterator(), "incoming" );
		if ( outgoing.size() > 0 )
			bs2 = checkArcs( outgoing.listIterator(), "outgoing" );
		if ( bs1 != null && bs2 != null )
			if ( !bs1.equals(bs2) )
				throw new MVDException("Incoming and outgoing sets not equal");
	}