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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_identifier(identifier, module_globals, module_name):
 """Helper utility to retrieve the callable function associated with a string identifier. Args: identifier: The identifier. Could be a string or function. module_globals: The global objects of the module. module_name: The module name Returns: The callable associated with the identifier. """

    if isinstance(identifier, six.string_types):
        fn = module_globals.get(identifier)
        if fn is None:
            raise ValueError('Unknown {}: {}'.format(module_name, identifier))
        return fn
    elif callable(identifier):
        return identifier
    else:
        raise ValueError('Could not interpret identifier')