code: public static WindowOver<Double> regrR2(Expression<? extends Number> arg1, Expression<? extends Number> arg2) {
          return new WindowOver<Double>(Double.class, SQLOps.REGR_R2, arg1, arg2);
      }
docs: REGR_R2 returns the coefficient of determination (also called R-squared or goodness of fit) for the regression.
  
  @param arg1 first arg
  @param arg2 second arg
  @return regr_r2(arg1, arg2)