human
I wrote the below code

    def f(rv):
        if (not (rv.func == tan)):
            return rv
        arg = rv.args[0]
        if arg.is_Add:
            if first:
                args = list(ordered(arg.args))
            else:
                args = list(arg.args)
            a = args.pop()
            b = Add._from_args(args)
            if b.is_Add:
                tb = TR12(tan(b), first=False)
            else:
                tb = tan(b)
            return ((tan(a) + tb) / (1 - (tan(a) * tb)))
        return rv
    return bottom_up(rv, f)

What's a good function header?
 Here's a good function header?
assistant
#Here's a good function header.

def TR12(rv, first=True):