main.js

define(function (require, exports, module) {
  'use strict';

  var Extensions = require('ft/core/extensions').Extensions;

  function test(editor, options, tagName) {
  	var collapseNodes = [];

    function fun(node) {
      var infos;
      if (node.firstChild && node.firstChild.line().trim() == ":INFOS:") {
        infos = node.firstChild;
      } else {
        infos = node.tree.createNode(":INFOS:");
        node.insertChildBefore(infos, node.firstChild)
      }
      var child = infos.tree.createNode("ADDED LINE");
      infos.insertChildBefore(child, infos.firstChild);
      collapseNodes.push(node);
    }

    var tree = editor.tree(),
        node;

    if (options && options.node) {
       node = options.node;
    } else {
       node = editor.selectedRange().startNode;
    }

    tree.beginUpdates();
    fun(node);
    tree.endUpdates();

    collapseNodes.forEach(function (each) {
    	editor.collapseNode(each);
    });
  }

  Extensions.addCommand({
    name: 'test',
    description: 'Test',
    performCommand: function (editor, options) {
      test(editor, options);
    }
  });

  Extensions.addInit(function (editor) {
    editor.addKeyMap({
      'Cmd-Alt-D' : 'test'
    });
  });

});

test.ft

- blah
- item
- blah

Put your cursor on `item` then press Cmd-Alt-D

I used `editor.collapseNode(infos);` at line 36, which is not in the API :-/