docs: def flatten(cls, errors):
          """ Unwind `MultipleErrors` to have a plain list of `Invalid`
  
          :type errors: list[Invalid|MultipleInvalid]
          :rtype: list[Invalid]
          """
          ers = []
          for e in errors:
              if isinstance(e, MultipleInvalid):
                  ers.extend(cls.flatten(e.errors))
              else:
                  ers.append(e)
          return ers