Imagine a sightseeing tour in the borough of Manhattan in New York City,
where a group of tourists are determined to walk from the corner of 59th
Street and 8th Avenue to the Chrysler Building at 42nd Street and Lexington
Avenue. There are many attractions along the way, but assume for the
moment that the tourists want to see as many attractions as possible. The
tourists are allowed to move either to the south or to the east, but even so,
they can choose from many different paths (exactly how many is left as a
problem at the end of the chapter). The upper path in figure 6.3 will take
the tourists to the Museum of Modern Art, but they will have to miss Times
Square; the bottom path will allow the tourists to see Times Square, but they
will have to miss the Museum of Modern Art.