The character

The (ascii: .) filter is a set filter that represents the 64 squares of the chessboard. It matches any position. The filter is equivalent to
      a-h1-8

For example, to check if the white queen attacks 27 squares, use

      ==27
Technically, this only works if there is at most one white on the board. To check that any white queen attacks 27 squares, use
      z z==27