code: 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);
    }
docs: Controls are recognized after the initial set of Rho blocks,
  before the generic `emitParagraph`.