Consider the following definition of the function subst:

1.  var subst = function (n,o,t) {
2.     if (fp.isNull(t)) {
3.         return t;
4.     } else if (fp.isList(fp.hd(t))) {
5.         return fp.cons(subst(n,o,fp.hd(t)),subst(n,o,fp.tl(t)));
6.     } else if (fp.isEq(o,fp.hd(t))) {
7.         return fp.cons(n,subst(n,o,fp.tl(t)));
8.     } else {
9.         return fp.cons(fp.hd(t),subst(n,o,fp.tl(t)));
10.    }
11. };

Suppose that you want a function that inserts its first argument before each occurrence of its second argument in its third argument, where that third argument must be a list. Hence a call with the arguments 4, 5, and [1,2,3,[4,5],5,6] would return [1,2,3,[4,4,5],4,5,6]. This could be accomplished by making a small change in one line of the subst function above and leaving the rest of the function unchanged. Which line should you change?

1. Line 4
2. Line 5
3. Line 6
4. Line 7
5. Line 9