.wherein(fieldname, collection, negate=False)

        Returns a new DataTable with rows only where the value at
        `fieldname` is contained within `collection`.

def wherein(self, fieldname, collection, negate=False):
        """
        .wherein(fieldname, collection, negate=False)

        Returns a new DataTable with rows only where the value at
        `fieldname` is contained within `collection`.
        """
        if negate:
            return self.mask([elem not in collection
                              for elem in self[fieldname]])
        else:
            return self.mask([elem in collection
                              for elem in self[fieldname]])