code: @Override
    public void exitNotExpression(GDLParser.NotExpressionContext ctx) {
      if (!ctx.NOT().isEmpty()) {
        Predicate not = new Not(currentPredicates.pop());
        currentPredicates.add(not);
      }
    }
docs: Called when we leave an NotExpression.
  
  Checks if the expression is preceded by a Not and adds the filter in that case.
  @param ctx expression context