filter promotion

    In a generate and test algorithm, combining part
   of the filter with the generator in order to reduce the number
   of potential solutions generated.  A trivial example:

   	filter (< 100) [1..1000]  ==>  [1..99]

   where [1..n] generates the list of integers from 1 to n.  Here
   the filter has been combined completely with the generator.
   This is an example of fusion.

   (2005-03-18)