public abstract class Progression {
	protected long first;
	protected long current;
	
	public Progression(long start)
	{
		current=first=start;
	}
	
	public long firstValue()
	// Reset the progression to the first value and return that value.
	{
		current=first;
		return current;
	}
	
	public abstract long nextValue();
	// Step the progression to the next value and return that value.
	
	public void printProgression(int length)
	// Print the first 'length' values of the progression. 
	{
		System.out.print(firstValue());
		for (int i=2; i<=length; i++)
			System.out.print(" "+nextValue());
		System.out.println();
	}
}