code: def list_formats(format_type, backend=None):
      
      if backend is None:
          backend = Store.current_backend
          mode = Store.renderers[backend].mode if backend in Store.renderers else None
      else:
          split = backend.split(':')
          backend, mode = split if len(split)==2 else (split[0], 'default')
  
      if backend in Store.renderers:
          return Store.renderers[backend].mode_formats[format_type][mode]
      else:
          return []
docs: Returns list of supported formats for a particular
      backend.