undo () {
      if (!els.length) return
      els.forEach(el => el.parentElement.removeChild(el))
      els = []
      if(!substitutedEls.length) return
      substitutedEls.forEach((el, idx) => {
        var parent = el.parentElement
        parent.replaceChild(originalSubstitutedEls[idx], el)
      })
      substitutedEls = []
      originalSubstitutedEls = []
    }
Undo elements to its previous state
@type {Function}