% return a line of consecutive positions in the north direction
% of other player's stones
% example:
% given
% S = ([[.,.,.,.,.,.], 
%       [.,.,.,.,.,.],
%       [.,.,2,1,.,.],
%       [.,.,1,2,.,.],
%       [.,.,.,2,.,.],
%       [.,.,.,x,.,.] ]).
%
%       x is at [3,4]
%
% northLine(S,1,[3,4],[[3,3],[3,2]]) holds
northLine(State,Plyr,Pos,[NorthOfPos|FollowingPositions]) :-
        otherPlyr(Plyr,OtherPlyr),
        north(Pos,NorthOfPos),
        get(State,NorthOfPos,OtherPlyr),
        northLine(State,Plyr,NorthOfPos,FollowingPositions).
northLine(State,Plyr,Pos,[]) :-
        otherPlyr(Plyr,OtherPlyr),
        north(Pos,NorthOfPos),
        get(State,NorthOfPos,PosContent),
        OtherPlyr \== PosContent.