"'certEnds' displays the list of identities , among the reference list 'group', who are MEMBER or (possibly) MISSING and about to loose their 'ParameterName.sigQty'th oldest certification, in the increasing order of event dates (bct); 'startFromNow' gives the period (in seconds) before the beginning of the list (0 if absent or null) , and 'period' gives the period covered by the list (infinite if absent or null). If 'group' is null or empty, the reference list is the whole Web Of Trust; if any string in 'group' is empty or doesn't refer to a known identity, it's not taken into account for the calculation, and, furthermore, if it is not empty and unknown, an error is raised"