code: function createIfDirective(dom, tag, attr) {
        return create(IfExpr).init(dom, tag, attr)
      }
docs: Create a new if directive
  @param   { HTMLElement } dom - if root dom node
  @param   { Tag } context - tag instance where the DOM node is located
  @param   { String } attr - if expression
  @returns { IFExpr } a new IfExpr object