change some 'obsolete' dates that I had set to 13-Dec-2018 to 31-Dec-2018; extra care should be taken before removing theorems with obsolete date 31-Dec-xxxx
change some 'obsolete' dates that I had set to 13-Dec-2018 to 31-Dec-2018; extra care should be taken before removing theorems with obsolete date 31-Dec-xxxx

metamath/set.mm,metamath/set.mm,metamath/set.mm