@mathjax/src
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    Variable APPENDConst

    APPEND: "[+]" = '[+]'

    Used to append an array to an array in default options E.g., an option of the form

    { name: {[APPEND]: [1, 2, 3]} }

    where 'name' is an array in the default options would end up with name having its original value with 1, 2, and 3 appended.