SWI-Prolog -- attvar/1

Documentation
- Reference manual
  - Constraint Logic Programming
    - Attributed variables
      - Attribute manipulation predicates
        
        attvar/1
        
        put_attr/3
        
        get_attr/3
        
        del_attr/2
- Packages

Availability:built-in

attvar(@Term)

Succeeds if Term is an attributed variable. Note that var/1 also succeeds on attributed variables. Attributed variables are created with put_attr/3.