Pairwise recursive

OK, this may seem like a stupidly simple question, but I''ve never come accross this term before, and it''s on one of my past exam papers. i.e. Two functions in a rule based, backward chaining system ''verify-fact'' and ''verify-antecedent'' Anyone know what "pairwise recursive" actually means? My guess is that it''s two coupled functions that call each other indirectly (as antecedents are found for rules and passed on to verify-antecedents and then their antecedents are passed to verify-fact etc etc) but I''ve no idea of the official definition of pairwise recursive. Anyone know?