[R] How to flatten a tree (based on list) to a certain depth?

[Peng Yu]

Suppose that I have the following list of lists of frames 'root'
(let's call it a 'tree' of frames). I want to flatten it to be a list
of frames. However, if I unlist(root), it will flatten the frames as
well. Is there a simply way to flatten the tree to certain depth?

aframe1=data.frame(x=1:3,y=1:3)
aframe2=data.frame(u=7:9,v=11:13)
aframe3=data.frame(p=3:5,q=6:8)

main1=list(sub1=aframe1, sub2=aframe2)
main2=list(sub3=aframe3)

root=list(main1=main1, main2=main2)

str(root)
unlist(root)