Dunno. The only programming rule of 3 I know is that, in a C++ class, you provide a destructor, a copy constructor, and an assignment operator. But that doesn

Dunno. The only programming rule of 3 I know is that, in a C++ class, you provide a destructor, a copy constructor, and an assignment operator. But that doesn't fit either, because it's not really algebra, and every grandma certainly does not know it, and it doesn't fit in a Lisp article anyway.