Imagine you are traveling and have just reached the bank of a muddy river, but there are no bridges or boats or anybody to help you to cross the river. Unfortunately, you do not know to swim. When you look around in this confused situation where there is no help available to you, you notice a sign board as shown in Figure 4-1:
![A978-1-4842-2514-1_4_Fig1_HTML.jpg](/api/v2/epubs/9781484225141/files/A978-1-4842-2514-1_4_Fig1_HTML.jpg)
Figure 4-1. A signson the bank of a river
The sign says, ...