(match
:pgn heijden.pgn     ;the name of the PGN file to look for studies
:output wcct8.pgn   ;the name of the result file, must be nonempty

:forany thematic [QRBN]
(position

:shift                                   ;e4 is any square
:sequence
(
  (position
    :wtm
    :nocheck
    [QRBN]e4                             ; the first thematic piece A
        :attackcount  a  ?e4 1 9         ; attacked
        :attackcount  A  ?e4 0           ; but not guarded


    :attackcount a  $thematic  0    ;the second thematic piece B not attacked
                                          ;as the most attractive possibility

    :movefrom    $thematic            ; but it moves
    :moveto     .                     ; to empty square, no capture
    :tagmatch thematic [QRBN]         ; and it is not pawn
  );end position


  (position
    :nocheck                         ; not check

    [QRBN]e4                         ; the first thematic piece A
    :attackcount  a  ?e4 1 9         ; still attacked
    :attackcount  A  ?e4 0 0         ; and still not guarded

    :attackcount a $thematic  1 9    ; the second piece B is now attacked,too
    :attackcount A $thematic  0      ; and not guarded, too
  );end position

 ); end sequence
);end position

); end match