Why doesn't the expression terminate?

gistfile1.clj

(defn segmento [left middle right all]
  (fresh [xs]
    (appendo left middle xs)
    (appendo xs right all)))


user> (run* [q] 
        (segmento [1 2] [3 4] [5 6] [1 2 3 4 5 6]))
(_.0)

user> (run* [q] 
        (segmento [1 2] [3 4] [5 6] q))
((1 2 3 4 5 6))

user> (run* [q] 
        (segmento [1 2] [3 4] q [1 2 3 4 5 6]))
((5 6))

user> (run* [q] 
        (segmento [1 2] q [5 6] [1 2 3 4 5 6]))
((3 4))

user> (run* [q] 
        (fresh [xs ys] 
          (segmento [1 2] xs ys [1 2 3 4 5 6]) 
          (== q [xs ys])))
([() (3 4 5 6)] 
 [(3) (4 5 6)] 
 [(3 4) (5 6)] 
 [(3 4 5) (6)] 
 [(3 4 5 6) ()])

;; Does not terminate.
;; user> (run* [q] (segmento q [3 4] [5 6] [1 2 3 4 5 6]))
; Evaluation aborted.

;; Changing the order of the clauses yields "expected" results:

(defn segmento [left middle right all]
  (fresh [xs]
    (appendo xs right all)
    (appendo left middle xs)))


user> (run* [q]
        (fresh [a b c]
          (segmento a b c [1 2 3 4 5 6])
          (== q [a b c])))
([() () [1 2 3 4 5 6]]
 [() (1) (2 3 4 5 6)]
 [(1) () (2 3 4 5 6)]
 [() (1 2) (3 4 5 6)]
 [(1) (2) (3 4 5 6)]
 [() (1 2 3) (4 5 6)]
 [(1 2) () (3 4 5 6)]
 [(1) (2 3) (4 5 6)]
 [() (1 2 3 4) (5 6)]
 [(1 2) (3) (4 5 6)]
 [(1 2 3) () (4 5 6)]
 [(1) (2 3 4) (5 6)]
 [() (1 2 3 4 5) (6)]
 [(1 2) (3 4) (5 6)]
 [(1 2 3) (4) (5 6)]
 [(1) (2 3 4 5) (6)]
 [(1 2 3 4) () (5 6)]
 [() (1 2 3 4 5 6) ()]
 [(1 2) (3 4 5) (6)]
 [(1) (2 3 4 5 6) ()]
 [(1 2 3) (4 5) (6)]
 [(1 2) (3 4 5 6) ()]
 [(1 2 3 4) (5) (6)]
 [(1 2 3) (4 5 6) ()]
 [(1 2 3 4 5) () (6)]
 [(1 2 3 4) (5 6) ()]
 [(1 2 3 4 5) (6) ()]
 [(1 2 3 4 5 6) () ()])

It's important to consider which one receives ground vars and which one does not

I'll understand what that means when I get my version of TRS :) (it's on its way!)