// List abstract class
template <class Elem> class List {
public:
  // Reinitialize the list.  The client is responsible for
  // reclaiming the storage used by the list elements.
  virtual void clear() = 0;
  // Insert an element at the front of the right partition.
  // Return true if successful, false if the list is full.
  virtual bool insert(const Elem&) = 0;
  // Append an element at the end of the right partition.
  // Return true if successful, false if the list is full.
  virtual bool append(const Elem&) = 0;
  // Remove the first element of right partition. Return
  // true if successful, false if right partition is empty.
  // The element removed is returned in the parameter.
  virtual bool remove(Elem&) = 0;
  // Place fence at list start, making left partition empty
  virtual void setStart() = 0;
  // Place fence at list end, making right partition empty
  virtual void setEnd() = 0;
  // Move fence one step left; no change if already at start
  virtual void prev() = 0;
  // Move fence one step right; no change if already at end
  virtual void next() = 0;
  // Return length of left partition
  virtual int leftLength() const = 0;
  // Return length of right partition
  virtual int rightLength() const = 0;
  // If pos or more elements are in the list, set the size
  // of left partition to pos and return true.  Otherwise,
  // do nothing and return false.
  virtual bool setPos(int pos) = 0;
  // Return in first parameter the first element  of the
  // right partition.  Return true if successful, false
  // if the right partition is empty.
  virtual bool getValue(Elem&) const = 0;
  // Print the contents of the list
  virtual void print() const = 0;
};