The for clause binds the variable $y to each address in turn.
for 子句依次将变量 $y 绑定到每个地址。
2
If a programmer wishes to address the A has_many B through=> [X, Y, Z] issue, they can specify the desired through association in their model class definition, and DrySQL will honour this.
如果开发者想解决A has_many B through=> [X, Y, Z]的问题，他们可以在模型类中定义想要的间接关联，并且DrySQL也希望你这么做。
3
Switch A does not have the MAC address of Router Y and will therefore flood the frame out its ports.