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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def norm_proj_path(path, build_module):
 """Return a normalized path for the `path` observed in `build_module`. The normalized path is "normalized" (in the `os.path.normpath` sense), relative from the project root directory, and OS-native. Supports making references from project root directory by prefixing the path with "//". :raises ValueError: If path references outside the project sandbox. """

    if path == '//':
        return ''

    if path.startswith('//'):
        norm = normpath(path[2:])
        if norm[0] in ('.', '/', '\\'):
            raise ValueError("Invalid path: `{}'".format(path))
        return norm

    if path.startswith('/'):
        raise ValueError("Invalid path: `{}' - use '//' to start from "
                         "project root".format(path))

    if build_module == '//':
        build_module = ''
    norm = normpath(join(build_module, path))
    if norm.startswith('..'):
        raise ValueError(
            "Invalid path `{}' - must remain inside project sandbox"
            .format(path))
    return norm.strip('.')