docs: public String conditionalExpression() throws RecognitionException {
          int first = input.index();
          exprParser.conditionalExpression();
          if ( state.failed ) return null;
  
          if ( state.backtracking == 0 && input.index() > first ) {
              // expression consumed something
              String expr = input.toString( first,
                                            input.LT( -1 ).getTokenIndex() );
              return expr;
          }
          return null;
      }