I don't "get" the first two. e^(iτ/2) = -1 can be translated as "half a full rotation is equivalent to a symmetry" (And a full rotation to identity (for usual

I don't "get" the first two. e^(iτ/2) = -1 can be translated as "half a full rotation is equivalent to a symmetry" (And a full rotation to identity (for usual objects), but this is kind of obvious...)