The . character

The .-filter is a set filter that represents all 64 squares of the chessboard. It is thus always true.

The .-filter can be used as a way to accept "any position" within the next/previous context:

    next (. Ra3)
matches a position where the white rook is on a3 following the current move.

As a set, the .-filter is useful where one explicitly wants to consider all squares. For example:

R. Becker
Olympia Dunyasi

countsquares 27 attack (Q .)

matches any position where the white queen attacks the maximum 27 squares.