docs: def validate(self, processes=1, fast=False):
          """Checks the structure and contents are valid. If you supply
          the parameter fast=True the Payload-Oxum (if present) will
          be used to check that the payload files are present and
          accounted for, instead of re-calculating fixities and
          comparing them against the manifest. By default validate()
          will re-calculate fixities (fast=False).
          """
          self._validate_structure()
          self._validate_bagittxt()
          self._validate_contents(processes=processes, fast=fast)
          return True