Parameter expressions as argument of previous()?

[casella]

In Section 16.4 I read the following statements:

A variable to which previous has been applied is called a clocked state variable.

Operator 16.6 previous

previous(u)

The input argument u is a component expression (definition 16.6) or a parameter expression.

I'm not sure I understand why u can be a parameter expression. How can a parameter expression be a clocked state variable, since it is by definition constant?

[HansOlsson]

That formulation is odd, but I don't see any major reason to change it.
What should be stated is that there are two possibilities for the argument with different consequences:

• A component expression, that variable is then a clocked state variable
• A parameter expression.

If you close at least one eye and squint you can see that it is just clarifying the existing text.