type 'a prompt 
val shift : 'a prompt -> int -> (('b -> 'a) -> 'a) -> 'b
val reset : 'a prompt -> int -> (unit -> 'a) -> 'a
val eval : ('a prompt -> 'a) -> 'a
(* 
   eval a build hierachical shift-reset control operators and pass them 
   as argument to a. 

   Both shift and reset takes an interger as first argument which
   determines their level in the hierarchy. Weaker control operators are
   ignored by higher ones.    The higher the interger, the
   weaker the control operator. 

   This reset 0 .... reset 1 ... reset 2 ... shift 1 will capture the
   context up to reset 1. 

 *)