* @param subGraph the directly opposite subgraph
	 * @param left true if we are doing the left subarc, otherwise the 
	 * right
	 */
	private void installSpecial( TreeMap<SpecialArc,Graph> specials, 
		SpecialArc special, Graph subGraph, boolean left ) throws MVDException
	{
		assert special.getFrom() != null && special.to != null;
		// this is necessary BEFORE you recalculate the MUM
		// because it will invalidate the special's location 
		// in the treemap and make it unfindable
		if ( specials.containsKey(special) )
			specials.remove( special );
		MUM best = computeBestMUM( subGraph, special );
		if ( best != null )
			specials.put( special, subGraph );
        //System.out.println("special="+special.toString());
	}