A person with a skateboard is sitting on a bench.
A skateboarder is sitting.
A person with a skateboard is a skateboarder, and sitting on a bench can be simplified to sitting.
exists e1 x1 x2 x3.(on(e1,x3) & n_bench(x3) & Actor(e1,x1) & v_sit(e1) & with(x1,x2) & n_skateboard(x2) & n_person(x1))
exists e1 x1.(Actor(e1,x1) & v_sit(e1) & n_skateboarder(x1))