Powerset

[Simon Marlow]

=20
> The following seems to be a faster version of powerset that delivers
> results strictly in the order of increasing cardinality (i.e., all
> sets of size 1 first, then of size 2, etc). It seems to run faster
> than any other ordered version of powerset posted so far. On GHCi,
> length $ powerset [1..22] is computed roughly 4 times faster than
> powerset3 given earlier. On Hugs, the powerset below also runs faster,
> with less memory consumption and in fewer GC cycles, up to a limit of
> 18 for the size of the input set.

Just a word of warning: using GHCi or Hugs for comparison benchmarking
will give unreliable results.  Use GHC with -O.

Cheers,
	Simon