--- /dev/null
+<%args>
+$param # name of the form field containing the classnum
+$field => $param # SQL field that must match that form field
+</%args>
+<%init>
+my @values = grep /^(\d+)$/, $cgi->param( $param );
+if (@values) {
+ return "COALESCE($field, 0) IN (" . join(',', @values) . ')';
+} else {
+ return;
+}
+</%init>