Tighten xyz sniffer

        >>> fname = get_test_fname('Si.cif')

        >>> XYZ().sniff(fname)

        False

        >>> fname = get_test_fname('not_a_xyz_file.txt')

        >>> XYZ().sniff(fname)

        False

        """

        try:

            self.read_blocks(list(file_prefix.line_iterator()))

            return True  # blocks read successfully

        except (TypeError, ValueError):

            return False

        except IndexError as e:

            if "pop from empty list" in str(e):

                # file_prefix ran out mid block with no other errors

                # assume the whole file is ok

            return True

            else:

                # some other IndexError - invalid input

        except (TypeError, ValueError, IndexError):

            return False

    def read_blocks(self, lines):

            n_atoms = None

            comment = None

            atoms = []

            try:

                n_atoms = int(lines.pop(0))

                comment = lines.pop(0)

                for _ in range(n_atoms):

                    position = [float(i) for i in atom[1:4]]

                    atoms.append(symbol + str(position))

                blocks.append({"number_of_atoms": n_atoms, "comment": comment, "atom_data": atoms})

            except IndexError as e:

                if "pop from empty list" in str(e) and blocks:

                    # we'll require at least one valid block

                    pass

                else:

                    raise

        return blocks

    def set_meta(self, dataset, **kwd):

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