function left_most_child (N node):node; (This function returns the left-most child of the node N. S If there are no children,a null node will be returned * function right_sibling (N:node):node; (If node N has a sibling node,it will be retured as the value of this S function.Otherwise,a null node will be returned * function null_node (N:node):boolean; (This function returns a true value if and only if N is a null node * B/S function cons_treen (E:elemtype;T1,T2,...,Tn:tree):tree; (This function constructs a tree with a node N as its root containing element E and the trees T1,T2,...,Tn as its children,in that order. B/S The trees T1,T2,...,Tn should not be used again after this operation to construct other trees.If copies of these subtrees are required,they can be constructed using the assign(T,T)operation.This is meant for space-efficiency so that this function need not copy these subtrees in order to make a new tree *S S B/S B/S