code: public static <T> WindowOver<T> nthValue(Expression<T> expr, Expression<? extends Number> n) {
          return new WindowOver<T>(expr.getType(), SQLOps.NTHVALUE, expr, n);
      }
docs: NTH_VALUE returns the expr value of the nth row in the window defined by the analytic clause.
  The returned value has the data type of the expr
  
  @param expr measure expression
  @param n one based row index
  @return nth_value(expr, n)