SWI-Prolog -- unknown/2

Documentation
- Reference manual
  - Built-in Predicates
    - Debugging and Tracing Programs
      - trace/0
      - tracing/0
      - notrace/0
      - notrace/1
      - debug/0
      - nodebug/0
      - debugging/0
      - spy/1
      - nospy/1
      - nospyall/0
      - leash/1
      - visible/1
      - unknown/2
      - style_check/1
- Packages

Availability::- use_module(library(edinburgh)).(can be autoloaded)

unknown(-Old, +New)

Edinburgh-Prolog compatibility predicate, interfacing to the ISO Prolog flag unknown. Values are trace (meaning error) and fail. If the unknown flag is set to warning, unknown/2 reports the value as trace.