code: function () {
                  var entities = [], e, delim;
  
                  do {
                      e = this.addition() || this.entity();
                      if (e) {
                          entities.push(e);
                          // operations do not allow keyword "/" dimension (e.g. small/20px) so we support that here
                          if (!peek(/^\/[\/*]/)) {
                              delim = $char('/');
                              if (delim) {
                                  entities.push(new(tree.Anonymous)(delim));
                              }
                          }
                      }
                  } while (e);
                  if (entities.length > 0) {
                      return new(tree.Expression)(entities);
                  }
              }
docs: Expressions either represent mathematical operations, or white-space delimited Entities.  1px solid black @var * 2