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A woman's corpse is found in the elevator of an old house. The police found out the body to be of Mrs. Blanka Tatírková, who worked in a newspaper stand where all the tenants were buying from. It was the victim's birthday that very day. At the time of the murder, she had the day's sales with her. The murder weapon, a knife, is found in the elevator shaft. Who murdered Mrs. Tatírková and why?