true if this is the only reference to this list instance; false otherwise.
Complexity: O(n).
auto sl = SList!int(24, 42); assert(sl.isUnique); { auto sl2 = sl; assert(!sl.isUnique); sl2.front = 0; assert(sl.front == 0); } // sl2 goes out of scope assert(sl.isUnique);
Check whether there are no more references to this list instance.