code: def stringify(x):
      
      result = []
  
      def go(key, val, format, meta):
          if key in ['Str', 'MetaString']:
              result.append(val)
          elif key == 'Code':
              result.append(val[1])
          elif key == 'Math':
              result.append(val[1])
          elif key == 'LineBreak':
              result.append(" ")
          elif key == 'SoftBreak':
              result.append(" ")
          elif key == 'Space':
              result.append(" ")
  
      walk(x, go, "", {})
      return ''.join(result)
docs: Walks the tree x and returns concatenated string content,
      leaving out all formatting.