private BitSet checkArcs( ListIterator<Arc> iter, String type ) 
		throws MVDException
	{
		BitSet bs = new BitSet();
		while ( iter.hasNext() )
		{
			Arc a = iter.next();
			BitSet v = a.versions;
			for ( int i=v.nextSetBit(0);i>=0;i=v.nextSetBit(i+1) )
			{
				if ( bs.nextSetBit(i)==i )
					throw new MVDException("Version "
						+i+" present twice in "+type+" set");
				bs.set( i );
			}
		}
		return bs;
	}