[GRASSLIST:4035] topological snap across layers

Hi,
I'd like to know if Grass is capable to make snap across layers.
I try to explane me.
I'd like to digitilize a vector that has part of its boundary coincident with 
linear or boundary components of an other vector.
It will be useful if, when I digitilige the boundary of the new vector along 
the boundary of the second vector that is in background, the result is that 
the two parts of the two vector are topolological coincident.

Is possible make it?
Thank you
Leonardo Lami