public void updateSheetReferences( Boundsheet newSheet )
	{
		for( int i = 0; i < expression.size(); i++ )
		{
			Object o = expression.elementAt( i );
			if( o instanceof PtgMemFunc )
			{
				PtgMemFunc pmf = (PtgMemFunc) o;
				Stack s = pmf.getSubExpression();
				for( int x = 0; x < s.size(); x++ )
				{
					Object ox = s.elementAt( x );
					if( ox instanceof PtgArea3d )
					{
						PtgArea3d p3d = (PtgArea3d) ox;
						p3d.setReferencedSheet( newSheet );
					}// do we have other types we need to handle here? (within memfunc)
				}
			}
			else if( o instanceof PtgArea3d )
			{// do we have other types we need to handle here? (outside memfunc);
				((PtgArea3d) o).setReferencedSheet( newSheet );
			}
			// do nothing
		}
		updatePtgs();
	}