3. (4 points) Write a Binary_Search_Tree member function Sum that returns the sum of the items in its tree. Assume that a default constructor and operator-+ are defined for the item type T. If the tree is empty, return the default constructed T item. (To call a default constructor without declaring a variable, just give the name of the default constructor with no arguments. E.g. return T) You should write a recursive helper function to do this. You also should assume that the code is in-lined into the template definition like the other member functions in bst.h