docs: def where(self, fieldname, value, negate=False):
          """
          Returns a new DataTable with rows only where the value at
          `fieldname` == `value`.
          """
          if negate:
              return self.mask([elem != value
                                for elem in self[fieldname]])
          else:
              return self.mask([elem == value
                                for elem in self[fieldname]])