//S = bitmask representation of available subset
//{1,2,3} ,S = 3 means only {2,3} are available since binary(3) = 0 1 1

for (int G = S; G != 0 ; G = (G - 1) & S)

This one liner generates subset of a subset in bit representation form. Would be grateful if anyone can give any intuition or explain the above line. Dont want to mug this up.