def source_start(base='<STR_LIT>', book_id='<STR_LIT>'):
repo_htm_path = "<STR_LIT>".format(book_id=book_id)<EOL>possible_paths = ["<STR_LIT>",<EOL>repo_htm_path,<EOL>"<STR_LIT>".format(book_id),<EOL>"<STR_LIT>".format(book_id),<EOL>"<STR_LIT>".format(book_id),<EOL>"<STR_LIT>".format(book_id),<EOL>]<EOL>for path in possible_paths:<EOL><INDENT>fullpath = os.path.join(base, path)<EOL>if os.path.exists(fullpath):<EOL><INDENT>return path<EOL><DEDENT><DEDENT>return None<EOL>
chooses a starting source file in the 'base' directory for id = book_id