title: How to represent negation in BNF?
body: <p>Does BNF or ABNF support negation. That is exclude certain members of the set?
  I did not see any such negation operator in its syntax.</p>
  
  <p>For example, suppose <code>S</code> is the set of all alphanumeric strings that are <em>not equal</em> to <code>"foo"</code>
  What is the BNF for <code>S</code>? </p>