I have to stop due to teaching duties. First thing to do during the next session : improve paragraph 1.2.2 in the style of 1.2.1. Then I have to finish this section. After that, it seems logical to me to write a section where I define the notion of 'weakly generating set of the k-cells' and 'basis of the k-cells' of a n-category and then a section where I explain how to compute colimits in nCat. Maybe the order should be 'section on basis' then 'section on generating cells' then 'section on colimits'.