The Free On-line Dictionary of Computing (30 December 2018):

NU-Prolog

   L. Naish, U Melbourne.  A Prolog with 'when' declarations, the
   successor to MU-Prolog.  Type-checked.  "NU-Prolog Reference
   Manual - Version 1.3", J.A. Thom et al eds, TR 86/10, U
   Melbourne (1988).  Available (but not free).  (See
   PNU-Prolog).  E-mail: .