In garden design, a ha-ha is a trench that forms a physical barrier, usually to keep out livestock, without interrupting the view or aesthetics of the garden. Before the advent of mechanical lawnmowers, the ha-ha—also called a sunk fence—ensured that grazing sheep trimmed outside grasses without destroying the lawn. In 2005, the Washington Monument was surrounded by a ha-ha for security purposes. How, according to English writer Horace Walpole, did the ha-ha supposedly get its name? Discuss