title: Empty string as base case in BNF recursion?
body: <p>I'm writing a small grammar as an exercise for class and my professor didn't really get too specific with what qualifies a legal BNF expression in terms.</p>
  
  <p>The BNF grammar is supposed to recognize strings in this form: AB, AABB, AAABBB, A...B... (general form: AnBn)</p>
  
  <p>So, what I got up to was writing:</p>
  
  <pre><code>&lt;S&gt; --&gt; A&lt;S&gt;B | ""
  </code></pre>
  
  <p>My simple yes/no question is whether or not this is legal in BNF, and if not why?</p>
  
  <p>&lt;...> represents a non-terminal, I have no idea if that's convention or what not</p>