As a couple of you noted, there was a problem with the assignment as
handed out (my apologies).  Corrections (which require changing two
lines) have been made in ass4.html and ass4.sml.  There is also a bug in
SML of NJ which prevents the code to compile as given; a workaround is
shown below.

--------------------------------------------------
In Question 3, the instantiation of type intset was in the wrong
place.  Please change the signature to leave type intset uninstantiated:

  signature INTSET =
  sig
    type intset  (* was: type intset = BoolSignal.signal *)

    val int : intset
    val nat : intset
    val empty : intset
    val union : intset * intset -> intset
    val intersect : intset * intset -> intset
    val difference : intset * intset -> intset

    val member : int * intset -> bool
  end;

Instead, the result signature of the IntSet functor should show the
intset implementation to allow a simple answer to question 5.

  functor IntSet (structure BS : BOOL_SIGNAL)
    :> INTSET where type intset = BS.signal (* was :> INTSET *)
  =
  struct
    type intset = BS.signal
    ...
  end;  (* functor IntSet *)

--------------------------------------------------
There is also a bug in SML of New Jersey which prevents the code
as given to compile.  To work around this bug occurrences of signature
instantiations of the form

  where type t = t1 and s = s1

should be replaced by

  where type t = t1 where type s = s1

Both of these forms are correct, so I have replaced the former by
the latter for the sake of those running SML/NJ.  If you are running
MLWorks, you may ignore this change.