PostFilters are designed to complement PostProcesses. While PostProcesses operate on the entire resultset at once, with PostFilters, each PostFilter is called for each result in the resultset. According to the return of filter() the result can then be included, discarded, or (un)boosted in the resultset. Possible return values for filter are FILTER_OK, FILTER_REMOVE, FILTER_ADJUSTED Which PostFilters are run, and when is controlled by two properties, as mentioned below.
'''NB:''' Initialisation and running of post filters is carried out by a subclass of the Manager.