SWI-Prolog -- project

Documentation
- Reference manual
  - Constraint Logic Programming
    - Attributed variables
      - Attributed variable hooks
        
        attr_unify_hook/2
        
        attribute_goals//1
        
        project_attributes/2
        
        attr_portray_hook/2
- Packages

Availability::- use_module(library(clpqr/project)).

project_attributes(+QueryVars, +ResidualVars)

A hook that can be defined in each module to project constraints on newly introduced variables back to the query variables. QueryVars is the list of variables occurring in the query and ResidualVars is a list of variables that have attributes attached. There may be variables that occur in both lists. If possible, project_attributes/2 should change the attributes so that all constraints are expressed as residual goals that refer only to QueryVars, while other variables are existentially quantified.