• f/: lhs := rhs 将 rhs 赋为lhs的延迟值，并对符号 f 关联这个赋值。
Nothing is done here.
This defines a rule for the product of Sin and Cos and associates that rule with the Sin function. Sin has to be unprotected first.
By storing the rule with Sin instead of the top level operation Times, you avoid slowing down every computation with Times.
We restore the original behavior of Sin. Clear does not get rid of the built-in definitions for Sin.