fn = () => {
  return {
    element: {
      enter: (node) => {
        if (node.name === 'defs') {
          /** @type {Map<string, number>} */
          const frequencies = new Map();
          for (const child of node.children) {
            if (child.type === 'element') {
              const frequency = frequencies.get(child.name);
              if (frequency == null) {
                frequencies.set(child.name, 1);
              } else {
                frequencies.set(child.name, frequency + 1);
              }
            }
          }
          node.children.sort((a, b) => {
            if (a.type !== 'element' || b.type !== 'element') {
              return 0;
            }
            const aFrequency = frequencies.get(a.name);
            const bFrequency = frequencies.get(b.name);
            if (aFrequency != null && bFrequency != null) {
              const frequencyComparison = bFrequency - aFrequency;
              if (frequencyComparison !== 0) {
                return frequencyComparison;
              }
            }
            const lengthComparison = b.name.length - a.name.length;
            if (lengthComparison !== 0) {
              return lengthComparison;
            }
            if (a.name !== b.name) {
              return a.name > b.name ? -1 : 1;
            }
            return 0;
          });
        }
      },
    },
  };
}
Sorts children of defs in order to improve compression. Sorted first by
frequency then by element name length then by element name (to ensure
grouping).

@author David Leston

@type {import('../lib/types.js').Plugin}