If I was writing my own function, I'd go with recursion because it'd result in the shortest code. I assume that that's what you're doing.
You're not saying what your trouble is for A+B+C, though. Are you getting List(List(A,B),List(C)) instead of List(A,B,C) ? If so, you should "add" or "dot" the two lists instead of doing whatever you're doing. (Source: the "Construction" section of List's wiki page)
You're not saying what your trouble is for A+B+C, though. Are you getting List(List(A,B),List(C)) instead of List(A,B,C) ? If so, you should "add" or "dot" the two lists instead of doing whatever you're doing. (Source: the "Construction" section of List's wiki page)