somebody in the air on a board
There is nobody on a board.
Either there is somebody on a board or not. They can not both be true.
exists x1 x2 x3.(n_air(x2) & a_topic(x1) & in(x1,x2) & on(x2,x3) & n_board(x3) & n_person(x1))
exists p1 x1.-exists x2 x3.((x1 = x2) & on(x2,x3) & n_board(x3) & n_person(x2))