default value with interactive

[Emanuel Berg]

I'm preparing one of my packs for MELPA so I go
thru the list of formal style
suggestions [1] ...

One I don't know how to do is when there is
a default value. The info node that is
referenced in the suggestion list, namely

    (info "(elisp) Programming Tips")

says the prompt string should look like this

    Enter the answer (default 42):

However then it says it should be implemented
like this

    (defun foo (pos)
      (interactive
        (list (if SPECIFIED SPECIFIED-POS)))
      (unless pos (setq pos DEFAULT-POS))
      ...)

Note that DEFAULT-POS in this example does not
appear in the `interactive' form! So how does
it appear in the prompt string at all?

And even if it did, how do you then get around
it without having to compute/fetch DEFAULT-POS
two times? One time for the prompt string, and
then again to set pos (in the abstract example
above)?

Is there a real-code example somewhere when
this is done the right way?

[1] https://github.com/melpa/melpa/blob/master/CONTRIBUTING.md

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