Stone Walls and Breastworks?

[Steel God]

If you're behind a stone wall and (attempt) to build breastworks, would you get the benefit of both?  Couldn't find a reference in the manual for it, and I'm thinking code is code, so maybe you would get the benefit of both, but from the common sense approach I'm thinking the wall is already better than any breastworks you can throw up so the game wouldn't give you anything for the effort.