docs: def dollarfy(x):
      """Replaces Math elements in element list 'x' with a $-enclosed string.
  
      stringify() passes through TeX math.  Use dollarfy(x) first to replace
      Math elements with math strings set in dollars.  'x' should be a deep copy
      so that the underlying document is left untouched.
  
      Returns 'x'."""
  
      def _dollarfy(key, value, fmt, meta):  # pylint: disable=unused-argument
          """Replaces Math elements"""
          if key == 'Math':
              return Str('$' + value[1] + '$')
          return None
  
      return walk(x, _dollarfy, '', {})