forest: sharing subprojects ( "library"-style) ?

[Gábor Farkas]

hi,

i have the following scenario:

i have 2 repositories, A and B.

i have 2 more repositories, that contain some help-functions. these 
repositories are called C and D.

i need to have this layout:

A-|
    --C
    --D


B-|
    --C
    --D

in other words, both A and B use the helper functions from C and D.

so i would like to maintain this as forests, but how am i supposed to 
"share" the C and D repositories between A and B?

thanks,
gabor