Somebody recalls a slightly different version than I.
FSR: Incidentally, knights really suck on b7, e.g., Soltis vs A J Goldsby, 1981, so driving your opponent's knight there tends to be a good thing. If you're defending the endgame of two bishops versus knight, disregard the above advice, since there the various "N2" squares (b7, g7, b2, and g2) are the key squares the knight should occupy. See P Popovic vs Korchnoi, 1984. (Computers proved 20 years ago that that ending is a theoretical win - though it's very difficult, see Timman vs Speelman, 1992.)
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Somebody recalls a slightly different version than I.