The selection of events prior to product extraction is carried out by the first stage of evselect. This processing step is driven by a boolean expression, i.e., events for which the expression evaluates to ``true'' are included in the subsequent product generation. The selection expression to be applied to the data is displayed in the xmmselect main window and is editable at all times. In addition, several GUI elements facilitate the construction of complex selection expression in a convenient manner (see below for more details).