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# Given a list of M items and a number N,
# generate all size-N subsets of M
...
    return ( (map {[$first, @$_]} @include_combos)
         , @exclude_combos );
}

sub iter_choose_n {
    my $n = pop;
...
    return sub {$once++ ? () : []} if $n == 0  or $n > @_;
    my ($first, @rest) = @_;
    return sub {$once++ ? () : [$first, @rest]} if $n == @_;

    # otherwise..
    my $include_iter = iter_choose_n(@rest, $n-1);
...
            return $exclude_iter->();
        }
    }

    # otherwise..
    my $include_iter = iter_choose_n(@rest, $n-1);
...
        }
    }
}

    # otherwise..
    my $include_iter = iter_choose_n(@rest, $n-1);
...
        }
    }
}