'Return a string version of this progress: either
"none", "in_progress", or "done".
subclassing note: implemented in terms of frac()'

def ternary_str(self):

    (a, b) = self.frac()
    if (a == 0):
        return 'none'
    if (a < b):
        return 'in_progress'
    return 'done'
