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: <jas@mulga.oz.au>.