"10Wheels" posted a picture of his method using mirrored sunglasses lenses fixed to the back of his cycle gloves. It works. I use that method now. Do remember this, if you cannot get a good feeling of the space behind you, ya better stop and take the time to turn and look around. There could be something really bad on your six...
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No matter where you're at... There you are... Δf:=f(1/2)-f(-1/2)