The nested lists are forming a parent-child relationship based on a set of keys.
嵌套列表基于一组键形成父子关系。
Visual user agents generally indent nested lists with respect to the current level of nesting.
可视化用户代理通常会对嵌套列表根据其嵌套的级别进行缩进。
Note that the inv function USES SPL nested lists to represent matrices, as there is no matrix type in SPL.
请注意，inv函数使用SPL嵌套列表来表示矩阵，因为SPL中没有矩阵类型。
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