<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def files_in_subdir(dir, subdir):
 """Find all files in a directory."""

    paths = []
    for (path, dirs, files) in os.walk(os.path.join(dir, subdir)):
        for file in files:
            paths.append(os.path.relpath(os.path.join(path, file), dir))
    return paths