has_vector D has_vector V = def for all d and all times t, if d instance_of D at t2, then there exists some v at t1 such that v instance_of V at t1 and d has_vector v at t2 and t1 earlier t2 has_vector not inverse of vector_of. D has_vector V = def for all d and all times t, if d instance_of D at t2, then there exists some v at t1 such that v instance_of V at t1 and d has_vector v at t2 and t1 earlier t2 relationship