@classmethod
def clearSize(cls, f0):
    """Return a copy of f0 with no size specified, or None if f0 is None."""
    if (f0 is None or f0.owner is None and f0.dataset is None and f0.mtime is
        None and f0.notPaths == [] and f0.regex is None):
        return None
    else:
        return cls(f0.owner, f0.dataset, None, f0.mtime, f0.notPaths, f0.regex)
