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