docs: def dialect(self):
          """File dialect as one of model.DIALECT_* constants
          """
          if self._dialect is None:
              self._dialect = model.DIALECT_DEFAULT
              if self.header:
                  source = self.header.sub_tag("SOUR")
                  if source:
                      if source.value == "MYHERITAGE":
                          self._dialect = model.DIALECT_MYHERITAGE
                      elif source.value == "ALTREE":
                          self._dialect = model.DIALECT_ALTREE
                      elif source.value == "ANCESTRIS":
                          self._dialect = model.DIALECT_ANCESTRIS
          return self._dialect