usedstdm nightmare!

This seems really stupid!

Im trying to select a part of a matrix, effect an area of the matrix and then output it back with the effected part mixed in . This patch works if I edit the matrix attributes directly in the object but doesnt if I try to send the same attributes as messages it doesnt do anything , I thinks its because of the usesrcdim usesdestdim flags but I cant think of a way around not using them!

Help!