Problem A - Bit Strings
-----------------------

Your task is to generate, in numerical order, all n-bit binary strings
that contain at most k 1's.  (Leading 0's are permitted.)

Input format
------------
There will be multiple input instances.  Each instance appears on a
separate line.  The line contains two numbers, n and k, separated by a
space.  For each instance, 0 <= k <= n.  The end of the input is
indicated by having k = n = 0.  (This last line of the input file
should not generate any output.) 

Output format
-------------
For each instance, list all strings in numerical order.

Sample input
------------
5 3
7 1
0 0

Sample output
-------------
00000
00001
00010
00011
00100
00101
00110
00111
01000
01001
01010
01011
01100
01101
01110
10000
10001
10010
10011
10100
10101
10110
11000
11001
11010
11100
0000000
0000001
0000010
0000100
0001000
0010000
0100000
1000000