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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 verify_fun(lazy_obj, fun):
    '''
    Check that the function passed really exists
    '''
    if not fun:
        raise salt.exceptions.SaltInvocationError(
            'Must specify a function to run!\n'
            'ex: manage.up'
        )
    if fun not in lazy_obj:
        # If the requested function isn't available, lets say why
        raise salt.exceptions.CommandExecutionError(lazy_obj.missing_fun_string(fun))