###
### interleave
###

## Chapter 7 section 3.1

sub interleave {
  my ($a, $b) = @_;
  return sub {
    my $next = $a->();
    unless (defined $next) {
      $a = $b;
      $next = $a->();
     }
    ($a, $b) = ($b, $a);
    $next;
  }
}