fnc

Define an anonymous constraint that can be used with the unifier:
    (let [oddc (fnc [x] (odd? x))]
      (unifier {:a '?a} {:a 1} :when {'?a oddc})
       ;;=> {:a 1}
      (unifier {:a '?a} {:a 2} :when {'?a oddc})
       ;;=> nil
   )
 Note, the constraint will not run until all arguments are fully ground.
 Use defnc to define a constraint and assign a toplevel var.