class ConsCell:
  def __init__(self, h, t):
    """Creates a new cell with head == h, and tail == t."""
    self.head = h
    self.tail = t

class List:
  """Describes a simple list data type."""

  def __init__(self, n):
    """creates a new list with n as the first element."""
    self.start = n

  def cons(self, e):
    """Adds a new element into the list; hence, procuding a new list."""
    return List(ConsCell(e, self.start))

  def length(self):
    """Returns the number of elements in the list."""
    len = 0
    cell = self.start
    while cell != 0:
       len += 1
       cell = cell.tail
    return len

  def __str__(self):
    """Returns a textual representation of this list."""
    ans = "["
    cell = self.start
    while cell != 0:
      ans = ans + repr(cell.head)
      cell = cell.tail
      if cell != 0:
        ans = ans + ", "
    ans = ans + "]"
    return ans

def test():
  a = List(0)
  b = a.cons(2)
  c = b.cons("Hi!")
  d = b.cons(True)
  e = d.cons(False)
  print "List a = ", a.__str__(), " Length(a) = ", a.length()
  print "List b = ", b.__str__(), " Length(b) = ", b.length()
  print "List c = ", c.__str__(), " Length(c) = ", c.length()
  print "List d = ", d.__str__(), " Length(d) = ", d.length()
  print "List e = ", e.__str__(), " Length(e) = ", e.length()

test()