[R] partitioning vectors of intervals

[Alexy Khrabrov]

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                [3,4,5]

Is there a functional way to do this?
Cheers,
Alexy