docs: def _check_exclude(self, val):
          """
          Validate the excluded metrics. Returns the set of excluded params.
          """
          if val is None:
              exclude = frozenset()
          elif isinstance(val, str):
              exclude = frozenset([val.lower()])
          else:
              exclude = frozenset(map(lambda s: s.lower(), val))
  
          if len(exclude - frozenset(METRICS)) > 0:
              raise YellowbrickValueError(
                  "'{}' is not a valid metric to exclude".format(repr(val))
              )
  
          return exclude