undivert to a file

On the GNU m4 home page it says to send suggestions for
m4 extensions here.

My suggestion is to be able to undivert to a file. So,

divert(1)
here is some text
divert(0)
undivertf(1, `filename.txt')

saves the diversion to the stated file, replacing
it. Additionally,

undivertf(1, `dir1/dir2/filename.txt')

would create dir1 and dir1/dir2 if they do not already exist.

I currently have obtained this function by programming a filter
on the back end of m4, but it would be nice to have it in m4
proper.