New version of rule for <munder>

从 default.yaml 中现有的 munder 朗读规则开始，将其与 issue 中描述的瑞典语规则进行比较。使用嵌套的 MathML 示例检查朗读是否更简洁，同时在导航过程中仍可使用花括号；当英语规则提供所提议的等效行为时，即表示完成。

In default.yaml there is a rule for speaking <munder>. In cases where there are nested <munder>s, such as the following,

<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML">
          <munder>
            <mrow>
              <munder>
                <mrow>
                  <mn>2</mn>
                  <mo>,</mo>
                  <mn>7</mn>
                  <mo>,</mo>
                  <mn>12</mn>
                  <mo>,</mo>
                  <mn>17</mn>
                  <mo>,</mo>
                  <mo>…</mo>
                  <mo>,</mo>
                  <mn>n</mn>
                  <mo>+</mo>
                  <mn>5</mn>
                </mrow>
                <mo>⏟</mo>
              </munder>
            </mrow>
            <mi mathvariant="normal">N</mi>
          </munder>
        </math>

the speaking is exceedingly verbose. In this case it would read "quantity quantity, 2 comma 7 [...] plus 5, with bottom brace below, with N below".

In Swedish, I have modified this rule so that it says the equivalent of "commented expression, 2 comma 7 [...] plus 5, with comment below, N, end comment". The bottom brace will be spoken only in navigation.

The new Swedish rule looks like this (translated into English here):

- name: default
  tag: munder
  match: "."
  replace:
  - with:
      variables:
      # true if the comment contains ⏟ or ︸ (also if inside mrow or surrounded by whitespace)
      - HasBrace: "contains(normalize-space(string(*[2])), '⏟') or contains(normalize-space(string(*[2])), '︸')"
      replace:
      - test:
          if: "$HasBrace"
          then:
            # The base is always read
            - x: "*[1]"
            - pause: short
            # Read the brace (good for navigation) but no framing phrases
            - x: "*[2]"
          else:
            # Frame phrase in the beginning if the base is not leaf
            - test:
                if: "not(IsNode(*[1], 'leaf'))"
                then: [T: "commented expression"]
            - x: "*[1]"
            - pause: short
            - T: "with comment below"
            - pause: short
            - x: "*[2]"
            - pause: short
            - T: "end comment"

If others agree that this is a better solution, I would suggest substituting the English rule for this.