Mine

(class Concatenate is NuOperator  
   (- (id) callWithArguments:(id)cdr context:(id)context is 
      (set final ((cdr car) evalWithContext: context))
      (set remaining (cdr cdr))
      (until (eq (remaining length) 0)
         (set current ((car remaining) evalWithContext: context))
         ; Append the current item onto the final sequence.
         (set final (final arrayByAddingObjectsFromArray: current))
         
         ; Drop the first element from the remaining ones.
         (set remaining (remaining cdr)))
      final))

(global ++ (Concatenate alloc))