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(emacs-lisp-intro.info)beginning-of-buffer opt arg

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`beginning-of-buffer' with an Argument
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   When `beginning-of-buffer' is called with an argument, an expression
is evaluated which calculates what value to pass to `goto-char'.  This
expression is rather complicated at first sight.  It includes an inner
`if' expression and much arithmetic.  It looks like this:

     (if (> (buffer-size) 10000)
         ;; Avoid overflow for large buffer sizes!
         (* (prefix-numeric-value arg) (/ (buffer-size) 10))
       (/
        (+ 10
           (*
            (buffer-size) (prefix-numeric-value arg))) 10))

Disentangle beginning-of-buffer

Large buffer case

Small buffer case