Is short_seq a subsequence of long_seq.

def is_subsequence(short_seq, long_seq):
  """Is short_seq a subsequence of long_seq."""
  if not short_seq:
    return True
  pos = 0
  for x in long_seq:
    if pos == len(short_seq):
      return True
    if short_seq[pos] == x:
      pos += 1
  if pos == len(short_seq):
    return True
  return False