7. (a) Draw the forest F represented by the list expression: (P(Q(C(D), E,W)), H(S(X), L)) Draw the binary tree B corresponding to the forest F using the natural corte- spondence between forests and binary trees. (4 marks) Without drawing the internal structure of the nodes, add in the threads to turn your binary tree B into a threaded binary tree with a head node. In what order will the nodes be visited if B is traversed in postorder? (3 marks) (b) Suppose F is a forest and that B is the threaded binary tree corresponding to F using the natural correspondence between forests and binary trees. For any given node X in the forest F, let pX denote a pointer to the corresponding node in the binary tree B. Write an algorithm that, given px, returns a pointer to the node in B correspond- ing to the leftmost sibling of X in F, unless X itself is the leftmost child of its parent, in which case it should return the value nil