old_contents: from stew.core import Sort, generator, operation
  from stew.matching import var
  from collections.abc import Sequence
  from ADT.types.relative import Z
  
  
  class List(Sort):
  
      def __init__(self, *args, **kwargs):
          if(len(args) == 1) and isinstance(args[0], Sequence):
              Sort.__init__(self)
  
              if len(args[0]) == 0:
                  self._generator = List.empty
              else:
                  self._generator = List.cons
                  self._generator_args = {'tail': List(args[0][:-1]), 'head': Z(args[0][-1])}
          else:
              Sort.__init__(self, **kwargs)
  
      @generator
      def empty() -> List:
          pass
  
      @generator
      def cons(tail: List, head: Z) -> List:
          pass
  
      @operation
      def __add__(tail: List, head: List) -> List:
          # x + [] = x
          if head == List.empty():
              return tail
          # [] + y = y
          if tail == List.empty():
              return head
          #  x + (y,c) = cons(x + y, c)
          if head == List.cons(tail=var.y, head=var.c):
              return List.cons(tail=tail + var.y, head=var.c)
  
      @operation
      def append(self: List, other: List) -> List:
          self = self + other
          return self
  
      def _as_list(self):
          if self._generator == List.empty:
              return ''
          elif self._generator == List.cons:
              if self._generator_args['tail'] == List.empty():
                  return str(self._generator_args['head'])
              else:
                  return self._generator_args['tail']._as_list() + "," + self._generator_args['head'].__str__()
  
      def __str__(self):
          return "List(" + self._as_list() + ")"
  
new_contents: from stew.core import Sort, generator, operation
  from stew.matching import var
  from collections.abc import Sequence
  from ADT.types.relative import Z
  
  
  class List(Sort):
  
      def __init__(self, *args, **kwargs):
          if(len(args) == 1) and isinstance(args[0], Sequence):
              Sort.__init__(self)
  
              if len(args[0]) == 0:
                  self._generator = List.empty
              else:
                  self._generator = List.cons
                  self._generator_args = {'tail': List(args[0][:-1]), 'head': Z(args[0][-1])}
          else:
              Sort.__init__(self, **kwargs)
  
      @generator
      def empty() -> List:
          pass
  
      @generator
      def cons(tail: List, head: Z) -> List:
          pass
  
      @operation
      def __add__(tail: List, head: List) -> List:
          # x + [] = x
          if head == List.empty():
              return tail
          # [] + y = y
          if tail == List.empty():
              return head
          #  x + (y,c) = cons(x + y, c)
          if head == List.cons(tail=var.y, head=var.c):
              return List.cons(tail=tail + var.y, head=var.c)
  
      def _as_list(self):
          if self._generator == List.empty:
              return ''
          elif self._generator == List.cons:
              if self._generator_args['tail'] == List.empty():
                  return str(self._generator_args['head'])
              else:
                  return self._generator_args['tail']._as_list() + "," + self._generator_args['head'].__str__()
  
      def __str__(self):
          return "List(" + self._as_list() + ")"
  
subject: remove append method