def add(I1, I2):
    """Calculate the addition of two intervals as another interval

    Keyword arguments:
    I1 -- the first interval given as a tuple (a, b)
    I2 -- the second interval given as a tuple (c, d)
    """
    (from1, to1) = I1
    (from2, to2) = I2
    return (from1 + from2,  to1 + to2)