code: function visitor(parent) {
      var index = -1
      var children = parent && parent.children
  
      if (!children) {
        throw new Error('Missing children in `parent` for `visitor`')
      }
  
      while (++index in children) {
        callback(children[index], index, parent)
      }
    }
docs: /* Visit `parent`, invoking `callback` for each child.