I was confused about it too, but understood that what is meant is, that the ‘half-silvered mirror rule’ is a rule that does two things at once, namely x: (x*1, x*i), so it’s a multi-valued operation.
I was confused about it too, but understood that what is meant is, that the ‘half-silvered mirror rule’ is a rule that does two things at once, namely x: (x*1, x*i), so it’s a multi-valued operation.