<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def add_vectors(vec_a, vec_b, periodic):
    '''Returns the sum of the points vec_a - vec_b subject 
       to the periodic boundary conditions.

    '''
    moved = noperiodic(np.array([vec_a, vec_b]), periodic)
    return vec_a + vec_b