text: import fym
  import fym.core
  
  
  class Link(fym.BaseEnv):
      def __init__(self):
          super().__init__()
          self.pos = fym.BaseSystem(shape=(3, 1))
          self.vel = fym.BaseSystem(shape=(3, 1))
  
  
  class Env(fym.BaseEnv):
      def __init__(self):
          super().__init__()
          self.links = fym.core.Sequential(
              **{f"link{i:02d}": Link() for i in range(5)}
          )
  
  
  env = Env()
  