/*
  Implement the classes:
  - Node
  - Number
  - Operator
  - Addition
  - Subtraction
  - Multiplication
  - Division

  Node should have the following (pure virtual) members:
  - double evaluate()
    + Calculate the expression stored in 'this'
  - void print(std::ostream& os)
    + print the expression represented in 'this'
  - void print_tree(std::ostream& os, int depth)
    + print a textual representation of the expression tree
  - Node* clone()
    + return a a new instance of the expression stored in 'this'
 */