--timeout option not always respected

[rafoo]

On the following example, Zipperposition does not respect the --timeout 2 option but fills all the available memory:

data list a := nil | cons a (list a).

def[infix "++"] append : pi a. list a -> list a -> list a where
  forall l. append nil l = l;
  forall hd tl l. append (cons hd tl) l = cons hd (append tl l).

def foreach : pi a. list a -> (a -> prop) -> prop where
  forall p. foreach nil p = true;
  forall hd tl p. foreach (cons hd tl) p = (p hd && foreach tl p).

goal forall a (l1 l2 : list a) p. foreach (append l1 l2) p = (foreach l1 p && foreach l2 p).

Called with zipperposition --timeout 2 bug.zf.