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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 get_architecture(arch=None):
 """Returns the definition for the specified architecture string. If no string is specified, the system default is returned (as defined by the PROCESSOR_ARCHITEW6432 or PROCESSOR_ARCHITECTURE environment variables). """

    if arch is None:
        arch = os.environ.get('PROCESSOR_ARCHITEW6432')
        if not arch:
            arch = os.environ.get('PROCESSOR_ARCHITECTURE')
    return SupportedArchitectureMap.get(arch, ArchDefinition('', ['']))