<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _watch_folder(folder, destination, compiler_args):
 """Compares "modified" timestamps against the "compiled" dict, calls compiler if necessary."""

    for dirpath, dirnames, filenames in os.walk(folder):
        for filename in filenames:
            # Ignore filenames starting with ".#" for Emacs compatibility
            if watched_extension(filename) and not filename.startswith('.#'):
                fullpath = os.path.join(dirpath, filename)
                subfolder = os.path.relpath(dirpath, folder)
                mtime = os.stat(fullpath).st_mtime
                
                # Create subfolders in target directory if they don't exist
                compiled_folder = os.path.join(destination, subfolder)
                if not os.path.exists(compiled_folder):
                    os.makedirs(compiled_folder)
                
                compiled_path = _compiled_path(compiled_folder, filename)
                if (not fullpath in compiled or
                    compiled[fullpath] < mtime or
                    not os.path.isfile(compiled_path)):
                    compile_file(fullpath, compiled_path, compiler_args)
                    compiled[fullpath] = mtime