Size k subsets
	/**
	 * Print all size k subsets of a given set
	 * 
	 * e.g.
	 * 
	 * 		{1, 2, 3, 4}	k = 2
	 * 
	 * 		{1, 2} {1, 3} {1, 4} {2, 3} {2, 4} {3, 4}
	 * 
	 * */