def add_vec3_vec3(va, vb):
    """Return the sum of given vec3 values.

    >>> va = [1,5,-2]
    >>> vb = [-10,4,6]
    >>> add_vec3_vec3(va, vb)
    [-9, 9, 4]

    """
    assert type(va) == list
    assert type(vb) == list

    vv = [ None for _ in range(3) ]
    for row in range(3):
        vv[row] = va[row] + vb[row]
    return vv