On this page:
http://www.swi-prolog.org/pldoc/doc_for?object=pengine_ask/3
The link to pengine_send/3 leads to page saying the following:
Sorry, No manual entry for pengine_send/3
Here is the target link associated with the above message:
http://www.swi-prolog.org/pldoc/man?predicate=pengine_send/3