1 =>
    a =>
        i =>
            level 3
        ii =>
            level 3
        iii =>
            level 3
    b =>
        i =>
            level 3
        ii =>
            level 3
        iii =>
            level 3
2 =>
    a =>
        level2
    b =>
        level2
3 =>
    level 1
4 =>
    level 1