least fixed point

    A function f may have many fixed points (x
   such that f x = x).  For example, any value is a fixed point
   of the identity function, (\ x . x).

   If f is recursive, we can represent it as

   	f = fix F

   where F is some higher-order function and

   	fix F = F (fix F).

   The standard denotational semantics of f is then given by
   the least fixed point of F.  This is the least upper bound
   of the infinite sequence (the ascending Kleene chain)
   obtained by repeatedly applying F to the totally undefined
   value, bottom.  I.e.

   	fix F = LUB bottom, F bottom, F (F bottom), ....

   The least fixed point is guaranteed to exist for a
   continuous function over a cpo.

   (2005-04-12)