(ldb/db-rel mothero parent child)
(ldb/db-rel fathero parent child)

(defn parento [parent child]
  (l/conde [(fathero parent child)]
           [(mothero parent child)]))

(defn ancestoro [ancestor child]
  (l/conde [(parento ancestor child)]

           ;; FIXME: this blows the stack
           ;; NOTE: this is an XOR rather than an inclusive OR
           ;;       maybe conde is not the right one?
           [(l/fresh [a]
              (ancestoro a child)
              (parento ancestor a))]))