code: function createRefDirective(dom, tag, attrName, attrValue) {
        return create(RefExpr).init(dom, tag, attrName, attrValue)
      }
docs: Create a new ref directive
  @param   { HTMLElement } dom - dom node having the ref attribute
  @param   { Tag } context - tag instance where the DOM node is located
  @param   { String } attrName - either 'ref' or 'data-ref'
  @param   { String } attrValue - value of the ref attribute
  @returns { RefExpr } a new RefExpr object