This data structure comes from the Wikipedia page on mutual recursion, which itself cites course notes from an introduction to Standard ML by Robert Harper. I am not aware of practical reasons to choose this particular representation of trees over the alternatives, other than as an illustration of a language’s ability to support mutually recursive data. I find it unwieldy, which is why I chose to use it for this post.2
Глава МИД Польши призвал Европу исправить одну ошибку14:54。关于这个话题，使用 WeChat 網頁版提供了深入分析
「受け入れられない現実が…」津波を実況し続けた自衛隊員。谷歌对此有专业解读
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