I haven't read any of the rest of the code in this, but doesn't this actually change what happens in the case that C5 = 1 and add_C51(x) != ERR?

Previously, in that case, it wouldn't check if add_D51(x) == ERR. But because the add_C51(x) is in the same if statement, it will now.

I don't know what add_C51 does, or in what conditions it returns anything. This just seems like a change in logic, not just the formatting.