How are extra copy constructor parameters used?

[Ali Çehreli]

The second item in the documentation mentions "any number of default 
parameters" when describing copy constructor syntax:

   https://dlang.org/spec/struct.html#struct-copy-constructor

1) I can't figure out how to use those extra parameters. For example, I 
can't find a special function name to call explicitly:

   S.__cctor(a, 42);  // No go

2) Unlike the examples there, I think the parameter should most usefully 
be defined as 'const' unless there is a special reason:

struct S {

   // const(S) instead of S:
   this(ref const(S) that) {
   }
}

Do you agree?

Thank you,
Ali