search for: t2_start

I have two pairs of time intervals: coarse- and fine-grained. They're components of their respective dataframes, looking like, coarse: endtime starttime 1 t1_end t1_start 2 t2_end t2_start ... fine: is the same, except that its intervals presumably fall into the coarse's enclosing ones. The problem is to partition the fine intervals into the coarse ones, adding a list to each row showing which fine intervals fall in it, like coarse: ... fine 1 [1,2] 2...