code: function selectorExists(sel) {
    utils.assertString(sel, 'selector');
  
    if (!this.__selectorsMap__) {
      var Normalizer = require('../visitor/normalizer')
        , visitor = new Normalizer(this.root.clone());
      visitor.visit(visitor.root);
  
      this.__selectorsMap__ = visitor.map;
    }
  
    return sel.string in this.__selectorsMap__;
  }
docs: Returns true if the given selector exists.
  
  @param {String} sel
  @return {Boolean}
  @api public