def _fill_naked_singles(self):
        """Look for naked singles, i.e. cells with ony one possible value.

        :return: If any Naked Single has been found.
        :rtype: bool

        """
        simple_found = False
        for i in utils.range_(self.side):
            for j in utils.range_(self.side):
                if self[i][j] > 0:
                    continue
                p = self._possibles[i][j]
                if len(p) == 1:
                    self.set_cell(i, j, list(p)[0])
                    self.solution_steps.append(self._format_step("NAKED", (i, j), self[i][j]))
                    simple_found = True
                elif len(p) == 0:
                    raise SudokuHasNoSolutionError("Error made! No possible value for ({0},{1})!".format(i + 1, j + 1))

        return simple_found