A boy chasing a baseball.
The boy is ambulatory.
Ambulatory means "relating to, or adapted for, walking," and therefore, if a boy is chasing a baseball, he is ambulatory, as chasing is a form of walking.
exists e1 x1 x2 x3.(a_topic(x1) & Theme(e1,x3) & Actor(e1,x2) & v_chase(e1) & n_baseball(x3) & (x1 = x2) & n_boy(x1))
exists s1 x1.(n_boy(x1) & Theme(s1,x1) & a_ambulatory(s1))