Goal:
  
  A : Type
  H : ReflectEq A
  l1 : list A
  l2 : list A
  l : list A
  call := split_prefix l1 l2 : (list A * list A * list A)%type
  ============================
   (is_prefix l l1 → is_prefix l l2 → is_prefix l (fst (fst call)))


DEBUG:Going to execute:
subst call