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]])
Returns a new DataTable with rows only where the value at
        `fieldname` == `value`.