if nonzero, two user-defined Virtuoso/PL functions will be
called when free-text data are updated, not the default system routine.
These functions are recognized by their special names:
<datatable>_<datafield>_INDEX_HOOK will be called to index
new documents and
<datatable>_<datafield>_UNINDEX_HOOK will be called to
remove obsolete index information about deleted documents.