%
% decide whether to move east, move, mark as having moved, and update boundaries
  move( 2:d+1, 1:d+1 ) = fish( 2:d+1, 1:d+1 ) & ...
          ( direction( 2:d+1, 1:d+1 ) == 1) & ( ~fish( 2:d+1, 2:d+2 ) );
  fish(2:d+1,2:d+1)= ( fish(2:d+1,2:d+1) & (~move(2:d+1,2:d+1) ) ) | ...
          move(2:d+1,1:d);
  direction(2:d+1,2:d+1) = direction(2:d+1,2:d+1) .* ( 1-move(2:d+1,1:d));
  updateboundaries;
  if dbug == 1, 
      subplot(221), plotfish;
      disp(['i= ',num2str(i),' just moved east']), keyboard, 
  end