old_contents: import collections
  import io
  
  from steel.fields.numbers import BigEndian
  from steel import fields
  from steel.chunks import base
  
  __all__ = ['Chunk', 'Form']
  
  
  class Chunk(base.Chunk):
      id = fields.String(size=4, encoding='ascii')
      size = fields.Integer(size=4, endianness=BigEndian)
      payload = base.Payload(size=size)
  
  
  class Form(base.Chunk, encoding='ascii'):
      tag = fields.FixedString('FORM')
      size = fields.Integer(size=4, endianness=BigEndian)
      id = fields.String(size=4)
      payload = base.Payload(size=size)
  
  
  
new_contents: import collections
  import io
  
  from steel.fields.numbers import BigEndian
  from steel import fields
  from steel.chunks import base
  
  __all__ = ['Chunk', 'ChunkList', 'Form']
  
  
  class Chunk(base.Chunk):
      id = fields.String(size=4, encoding='ascii')
      size = fields.Integer(size=4, endianness=BigEndian)
      payload = base.Payload(size=size)
  
  
  class ChunkList(base.ChunkList):
      def __init__(self, *args, **kwargs):
          # Just a simple override to default to a list of IFF chunks
          return super(ChunkList, self).__init__(Chunk, *args, **kwargs)
  
  
  class Form(base.Chunk, encoding='ascii'):
      tag = fields.FixedString('FORM')
      size = fields.Integer(size=4, endianness=BigEndian)
      id = fields.String(size=4)
      payload = base.Payload(size=size)
  
  
  
subject: Customize a ChunkList just for IFF chunks