code: 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_f 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
docs: Returns the sum of the points vec_a - vec_b subject 
         to the periodic boundary conditions.