'Add two progress indicators, assuming that each represents items done:
(a / b) + (c / d) = (a + c) / (b + d).
If either is None, returns the other.'

@staticmethod
def add_counts(a, b):

    if (a is None):
        return b
    if (b is None):
        return a
    (n, d) = a.frac()
    (n2, d2) = b.frac()
    return Progress((n + n2), (d + d2))
