Make completing-read pick item after initial

[Heime]

How can I make completing-read start from an initial "Gramma", but rather than
starting from "Greek" as the next item, it continues with the next one, namely
"Flokki".  But I would like to retain the order of cseq as "Greek"  "Gramma"  
"Flokki" rather than changing it.

(defun avus-latex (seltr)
  "Display the full documentation of `avus-ams' tools."

  (interactive
    (list
      (let ( (cseq '("Greek"  "Gramma"  "Flokki")) )
        (completing-read "Doc: " cseq nil t "Gramma"))))

  (describe-function (pcase seltr
                       ("Greek"   'avus-latex-greek)
                       ("Gramma"  'avus-latex-gramma)
                       ("Flokki"  'avus-latex-flokki))) )