code: function() {
                  var e, entities = [];
  
                  e = $(this.addition);
                  while (e || $(this.entity)) {
                      entities.push(e);
                      e = $(this.addition);
                  }
  
                  if (entities.length > 0) {
                      return new tree.Expression(entities);
                  }
              }
docs: Expressions either represent mathematical operations, or white-space delimited Entities.  @var * 2