_applyTransform (params, transformFn) {
    const editorSession = params.editorSession
    const sel = params.selection
    if (sel.isNull()) return
    let result // to store transform result
    editorSession.setSelection(sel)
    editorSession.transaction(function (tx) {
      const out = transformFn(tx, params)
      if (out) result = out.result
    })
    return result
  }
Apply an annotation transformation.

    @returns {Object} transformed annotations.