Introduction to Modeling in Maya

[Shawty]

Hello! At the moment i'm doing the Introduction to Modeling in Maya Course and i just got stuck by a problem in the issue 23:Modeling the seat springs. The thing is they are doing boleans on nurbs, well i do exactly the same thing but Maya comes with a warning telling me: '//Warning: Operation has history, inputs are not deleted´, and the problem i actually delete the history, freeze transformations everything,when this problem shows up it actually do the bolean but some faces are deleted and the real ones who i want to be deleted don't, i search about this and i made like all i found, if somebody can help it's very much appreciate it, thanks in advance!