[1 2 3],[4 5 6]               :" join "
[1 2 3 4 5 6]

    [1 2],:\[[3 4] [5 6] [7 8]]   :" join each-left "
[[1 2 3 4] [1 2 5 6] [1 2 7 8]]

    [1 2],:/[[3 4] [5 6] [7 8]]   :" join each-right "
[[3 4 1 2] [5 6 1 2] [7 8 1 2]]