Complexity: k + m, where k is the number of elements in r and m is the length of stuff.
Example:
import std.algorithm, std.container, std.range;
auto sl = SList!string(["a", "b", "d"]);
sl.insertAfter(sl[], "e"); // insert at the end (slowest)
assert(equal(sl[], ["a", "b", "d", "e"]));
sl.insertAfter(take(sl[], 2), "c"); // insert after "b"
assert(equal(sl[], ["a", "b", "c", "d", "e"]));