Stateful streams. Define a stream data type that does not use dataflow variables. That is, it is a list in which each tail is a cell whose content points to the rest of the list. The last cell contains a marker saying that the stream is not yet complete, e.g., the atom incomplete.(This is not the same as the atom nil which means that the stream is complete.) There is a global cell C whose content is always the last cell in the stream. Write an operation that adds an element to the stream and that works in a concurrent setting. Hint: assume that there exists a statement lock S end such that only one thread at a time can be executing S; all others suspend if needed to make this true. Can you do it without using a lock statement? Compare your solution to that of the previous exercise. Which is simpler?