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).
