src/Pure/seq.ML

 (*return next sequence element as None or Some (x, xq)*)
 fun pull (Seq f) = f ();
 
 
 (*the empty sequence*)
-val empty = make (fn () => None);
+val empty = Seq (fn () => None);
 
 (*prefix an element to the sequence -- use cons (x, xq) only if
   evaluation of xq need not be delayed, otherwise use
   make (fn () => Some (x, xq))*)
 fun cons x_xq = make (fn () => Some x_xq);