title: Number of sub-sequences in a given sequence
body: <p>If I am given a sequence <code>X = {x1,x2,....xm}</code>, then I will have <code>(2^m)</code> subsequences.
  Can anyone please explain how can I arrive at this formula intuitively?
  I can start with 3 elements, then 4 and then 5 and arrive to this formula, but I don't think I understand. Where did the '2' come from? I am not dividing in half or anything here.
  Thank-you for the help.</p>