sealed rather than final so that UrlAttrName can add the one arm the url bearing names need, and no view file can invent a fourth kind of attribute name from outside this file.
A present attribute is the truth, so true renders the bare name and false renders nothing at all. The Iterable arm of Mod absorbs both, which is why a conditional attribute needs no helper of its own.
A present attribute is the truth, so true renders the bare name and false renders nothing at all. The Iterable arm of Mod absorbs both, which is why a conditional attribute needs no helper of its own.
