// Binary tree node abstract class
template <class Elem> class BinNode {
public:
  // Return the node's value
  virtual Elem& val() = 0;

  // Set the node's value
  virtual void setVal(const Elem&) = 0;

  // Return the node's left child
  virtual BinNode* left() const = 0;

  // Set the node's left child
  virtual void setLeft(BinNode*) = 0;

  // Return the node's right child
  virtual BinNode* right() const = 0;

  // Set the node's right child
  virtual void setRight(BinNode*) = 0;

  // Return true if the node is a leaf, false otherwise
  virtual bool isLeaf() = 0;
};