Controls are recognized after the initial set of Rho blocks,
before the generic `emitParagraph`.
function(walk) {
    var _super = BlockCompiler.prototype.emitParagraph.bind(this);
    if (this.tryGroup('checkbox', walk)) return;
    if (this.tryGroup('radio', walk)) return;
    if (this.trySortableGroup(walk)) return;
    if (this.tryAssociativeGroup(walk)) return;
    return _super(walk);
  }