<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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]])