This number line, and the two rows of inequalities below it, show how
the textbook derives the tests in the succession of else-if clauses.


  |-----------|---------------|---------------|---------------|---------------  Richter scale
  0          4.5             6.0             7.0             8.0

     default      r >= 4.5        r >= 6.0        r >= 7.0        r >= 8.0      (working right-to-left)

     r < 4.5      r < 6.0         r < 7.0         r < 8.0         default       (working left-to-right)



Here is the number line for the textbook's 24-hour clock example.

       night         morning        afternoon      evening
  |--------------|--------------|--------------|--------------|   hours
  0              6              12             18
     hour < 6       hour < 12       hour < 18       default       (working left-to-right)

     default        hour >= 6      hour >= 12       hour >= 18    (working right-to-left)


    // working left-to-right                 // working right-to-left
    if (hour < 6)                            if (hour >= 18)
    {                                        {
       time = "night";                          time = "evening";
    }                                        }
    else if (hour < 12)                      else if (hour >= 12)
    {                                        {
       time = "morning";                        time = "afternoon";
    }                                        }
    else if (hour < 18)                      else if (hour >= 6)
    {                                        {
       time = "afternoon";                      time = "morning";
    }                                        }
    else                                     else
    {                                        {
       time = "evening";                        time = "night";
    }                                        }