Its probably her job to check and likely a requirement to keep the library open to the public.
I get that but she was so nasty.
That's the problem with the way much of this has been handled.
State or local government directs businesses to enforce mask rules at risk of closure.
Business owner or manager, fearing closure, directs employee to stand by the door and enforce mandates with patrons. Employee, fearing loss of job, stands by the door and does as told by boss.
The patron being told to wear a mask is being confronted by somebody who probably has no desire whatsoever to enforce a mandate, but every desire in the world to keep their job. I'm sure there are exceptions and some people who are mask crusaders, but I don't think, since last spring, I've been told to put on a mask by anybody who really wouldn't have rather been doing something else.