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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 register_compressor(ext, callback):
 """Register a callback for transparently decompressing files with a specific extension. Parameters ext: str The extension. callback: callable The callback. It must accept two position arguments, file_obj and mode. Examples -------- Instruct smart_open to use the identity function whenever opening a file with a .xz extension (see README.rst for the complete example showing I/O):
 """

    if not (ext and ext[0] == '.'):
        raise ValueError('ext must be a string starting with ., not %r' % ext)
    if ext in _COMPRESSOR_REGISTRY:
        logger.warning('overriding existing compression handler for %r', ext)
    _COMPRESSOR_REGISTRY[ext] = callback