Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
In Canto Two of his poem John Shade (the poet in VN's novel Pale Fire, 1962) speaks of his dead daughter and mentions old Pan:
Another winter was scrape-scooped away.
The Toothwort White haunted our woods in May.
Summer was power-mowed, and autumn, burned.
Alas, the dingy cygnet never turned
Into a wood duck. And again your voice:
"But this is prejudice! You should rejoice
That she is innocent. Why overstress
The physical? She wants to look a mess.
Virgins have written some resplendent books.