/* Def. includes	
	 * Let #(x,s) = the number of occurrences of the charcter x in the string s.
	 * includes(a,b) holds if for every character x in b, #(x,b) <= #(x,a)
	 * 
	 * A necessary precondition for includes is that both strings are sorted
	 * in ascending order.
	 */
	private boolean includes( String a, String b ) {
		if ( b == null || b.length() == 0 )
			return true;
		else if ( a == null || a.length() == 0 )
			return false;
		
		//precondition: a.length() > 0 && b.length() > 0
		int i = 0, j = 0;
		while ( j < b.length() ) {
			if (i >= a.length() || b.charAt(j) < a.charAt(i))
				return false;
			else if (b.charAt(j) == a.charAt(i)) {
				i++; j++;
			} else if (b.charAt(j) > a.charAt(i))
				i++;
		}
		//postcondition: j == b.length()
		return true;
	}