People walk down a beach.
People walk at a beach
Walk at a beach is a rephrasing for walk down a beach.
exists e1 x1 x2.(down(e1,x2) & n_beach(x2) & Actor(e1,x1) & v_walk(e1) & n_people(x1))
exists e1 x1 x2.(at(e1,x2) & n_beach(x2) & Actor(e1,x1) & v_walk(e1) & n_people(x1))