How to use the pymoca.tree function in pymoca

if options.flatten_only:
    # Load folder
    _ast = None
    for root, dir, files in os.walk(model_folder, followlinks=True):
        for item in fnmatch.filter(files, "*.mo"):
            logger.info("Parsing {}".format(item))

            with open(os.path.join(root, item), 'r') as f:
                if _ast is None:
                    _ast = parser.parse(f.read())
                else:
                    _ast.extend(parser.parse(f.read()))

    logger.info("Flattening")

    _ast = tree.flatten(_ast, ast.ComponentRef.from_string(model_name))
    print(_ast)
else:
    # Set CasADi installation folder
    if options.casadi_folder is not None:
        sys.path.append(options.casadi_folder)

    from pymoca.backends.casadi.api import transfer_model
    import casadi as ca

    logger.info("Generating CasADi model")
    
    compiler_options = \
        {'replace_constants': True,
         'replace_parameter_expressions': True,
         'eliminable_variable_expression': r'_\w+',
         'detect_aliases': True,