Iterator Example

        my $it = make_iterator('foo-#-bar#');

        for (1..105) {
          my $s = $it->();
          print "$s\n";
        }

        # Prints foo-0-bar0, foo-0-bar1, 
        #        ..., 
        #        foo-9-bar8, foo-9-bar9, ...

Easy change to make it stop and return undef instead of starting over.
Construct many iterators that all operate independently.
Pass iterators to functions, store in data structures.