// From the textbook.  

// Note: This is slightly different from the binary tree interface you
// implemented in HW5, which did not use hasLeft() and hasRight().

/**
 * An interface for a binary tree, where each node can have zero, one,
 * or two children.
 */
public interface BinaryTree extends Tree {

  /** Returns the left child of a node. */
  public Position left(Position v) 
    throws InvalidPositionException, BoundaryViolationException;

  /** Returns the right child of a node. */
  public Position right(Position v) 
    throws InvalidPositionException, BoundaryViolationException;

  /** Returns whether a node has a left child. */
  public boolean hasLeft(Position v) throws InvalidPositionException;

  /** Returns whether a node has a right child. */
  public boolean hasRight(Position v) throws InvalidPositionException;
}