A homeless man was arrested this week at the downtown Central Library after allegedly masturbating in a back aisle and then pulling a knife on a librarian who was trying to escort him out of the building. A security guard used a Taser on the man, who was arrested by police and charged with multiple offenses, from lascivious conduct to aggravated battery. No one, luckily, was injured or killed in the incident. But it's an ugly reminder of a community problem hidden in plain sight. On any given day, you'll find a group of homeless regulars at the Central Library,...