code: function reference(node, index, parent) {
      var definition = definitions(node.identifier)
      var replacement
      var image
  
      if (definition) {
        image = node.type === 'imageReference'
  
        replacement = {
          type: image ? 'image' : 'link',
          url: definition.url,
          title: definition.title
        }
  
        if (image) {
          replacement.alt = node.alt
        } else {
          replacement.children = node.children
        }
  
        parent.children[index] = replacement
      }
    }
docs: /* Transform a reference based on bound `definitions`.