What I've got is a classic white picket fence in a retaining wall. (I live on the side of a hill near Puget Sound WA.) The top 1/3 of the corner post is rotten and needs to be replaced. It's pretty much impossible to cut the concrete of the retaining wall and recast the concrete, so I’m thinking my only option is to splice a new 4x4 to the top of the old post if I can make it accurate enough.