class Habitat:
  def __init__(self):
    self.name = ''
    self.members = set()

  def add_member(self, member):
    self.members.add(member)

  def remove_member(self, member):
    self.members.discard(member)



class Aquarium(Habitat):
  def __init__(self, water_type):
    Habitat.__init__(self)
    self.water_type = water_type
