sub enumerate {
  # base case: return an array containing a single
  # empty anonymous hash (ref).
  return ( +{} ) unless @_;

  # general case
  my %p = @_;
  my ( $key, $val ) = each %p;
  my @ret;
  delete $p{ $key };
  for my $h ( enumerate( %p ) ) { # recursion
    for my $d ( @$val ) {
      my %c = %$h;                # copy hash
      $c{ $key } = $d;
      push @ret, \%c;
    }
  }
  return @ret;
}