* @return float fraction of version that is unique
	 */
	private float getPercentUnique( short version )
	{
		float unique=0.0f,shared=0.0f;
		for ( int i=0;i<pairs.size();i++ )
		{
			Pair p = pairs.get( i );
			if ( p.versions.nextSetBit(version)==version )
			{
				if ( p.versions.size()==1 )
					unique += p.length();
				else
					shared += p.length();
			}
		}
		return unique/shared;
	}