People are waiting by the curb.
People are boarding the bus at the curb.
People are waiting or boarding, not both.
exists e1 x1 x2.(n_curb(x2) & by(e1,x2) & Actor(e1,x1) & v_wait(e1) & n_people(x1))
exists e1 x1 x2 x3.(n_curb(x3) & n_bus(x2) & at(e1,x3) & Theme(e1,x2) & Actor(e1,x1) & v_board(e1) & n_people(x1))