docs: 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