using xml** Conveys information about a category associated with an `Entry` or `Feed`.** ** The [atom:category]`http://tools.ietf.org/html/rfc4287#section-4.2.2` Elementclass Category {** *(Required)* ** Identifies the category. Str term** *(Optional)*** Identifies the categorization scheme via a URI. Uri? scheme** *(Optional)*** Provides a human-readable label for display. Str? label** Creates a 'Category' with the required fields.new make(Str term){this.term = term}** Creates a '<category>' element.virtual XElem toXml(){ category := XElem("category") category.addAttr("term", term)if(scheme != null) category.addAttr("scheme", scheme.toStr)if(label != null) category.addAttr("label", label)return category}}