docs: def genomic_signal(fn, kind):
      """
      Factory function that makes the right class for the file format.
  
      Typically you'll only need this function to create a new genomic signal
      object.
  
      :param fn: Filename
      :param kind:
          String.  Format of the file; see
          metaseq.genomic_signal._registry.keys()
      """
      try:
          klass = _registry[kind.lower()]
      except KeyError:
          raise ValueError(
              'No support for %s format, choices are %s'
              % (kind, _registry.keys()))
      m = klass(fn)
      m.kind = kind
      return m