docs: function Entity(o) {
          o = o || {};
  
          this.content = 'undefined' !== typeof o.content ? o.content : '';
  
          if ('string' === typeof o.id) {
              this.id = o.id;
          }
          else if ('undefined' !== typeof o.id) {
              throw new TypeError('Entity: id must ' +
                                  'be string or undefined.');
          }
          if ('string' === typeof o.className) {
              this.className = o.className;
          }
          else if (J.isArray(o.className)) {
              this.className = o.join(' ');
          }
          else if ('undefined' !== typeof o.className) {
              throw new TypeError('Entity: className must ' +
                                  'be string, array, or undefined.');
          }
      }