sub evens_up_to {
        odds_up_to( $_[0] + 1 )
            ->seq_map( sub { $_[0] - 1 } );
    }

    enumerate( evens_up_to(10) );

    #  0 => 0
    #  1 => 2
    #  2 => 4
    #  3 => 6
    #  4 => 8
    #  5 => 10