Why does the following Backus-Naur form reflect the following syntax diagram?
&lt; number&gt;::=&lt; digit &gt; | &lt; digit &gt;&lt; number &gt; means that a number is either just a digit, or a digit followed by a number. So 1 is a number (just a single digit), and 12 is also a number (a single digit, followed by a number that consists of just the single digit '2'). Similarly 123 is a number that consists of a single digit '1' and a number (that itself consists of a single digit '2' followed by a number (that consists of the single digit '3')).