def _raise_if_wrong_file_signature(stream):
file_sig = stream.read(len(headers.LAS_FILE_SIGNATURE))<EOL>if file_sig != headers.LAS_FILE_SIGNATURE:<EOL><INDENT>raise errors.PylasError(<EOL>"<STR_LIT>".format(file_sig, headers.LAS_FILE_SIGNATURE)<EOL>)<EOL><DEDENT>
Reads the 4 first bytes of the stream to check that is LASF