SWI-Prolog -- current_format_predicate/2

Documentation
- Reference manual
  - Built-in Predicates
    - Formatted Write
      - Programming Format
        
        format_predicate/2
        
        current_format_predicate/2
- Packages

Availability:built-in

current_format_predicate(?Code, ?:Head)

True when ~Code is handled by the user-defined predicate specified by Head.