Tactic from Tal, Montreal 1979

My mind was blocked on this puzzle, although it is quite straightforward.

White to play and gain a piece...yes, it is that simple, so why it took me a few minutes to solve it, I can only wonder !  


Finding out that this was actually from a game played by Tal against Larsen in Montreal 1979, made me look up the whole thing.  


Well worth the meagre effort to find it.


I would assume that Tal saw the whole combination from here, at move 16/17

...all the way to the end.  


Play through and enjoy it.