@Override
	public void close()
	{
		externsheet = null;
		if( ptga != null )
		{
			if( ptga instanceof PtgRef )
			{
				ptga.close();
			}
			else
			{
				ptga.close();
			}
			ptga = null;
		}
		if( expression != null )
		{
			while( !expression.isEmpty() )
			{
				GenericPtg p = (GenericPtg) expression.pop();
				if( p instanceof PtgRef )
				{
					p.close();
				}
				else
				{
					p.close();
				}
				p = null;
			}
		}
		super.close();
	}