1   2   3   4   5   6   7   8   9
           -----  --- --- --- --- --- --- --- --- ---
[1] start  1      XXX
[2] start  1 & 2  XXX XXX
[2] done   1      XXX 
[3] start  1 & 3  XXX     XXX
[3] done   1      XXX 
[4] start  1 & 4  XXX         XXX
[4] done   1      XXX 
[5] start  1 & 5  XXX             XXX
[5] done   1      XXX 
[1] done
[6] start  6                          XXX
[7] start  6 & 7                      XXX XXX
[6] done   7                              XXX
[7] done
[8] start  8                                  XXX
[9] start  8 & 9                              XXX XXX
[8] done   9                                      XXX
[9] done