def letterSequence(letters, w=40):
  """
  Return a list of input vectors corresponding to sequence of letters.
  The vector for each letter has w contiguous bits ON and represented as a
  sequence of non-zero indices.
  """
  sequence = []
  for letter in letters:
    i = ord(letter) - ord('A')
    sequence.append(set(range(i*w,(i+1)*w)))
  return sequence