code: public static WindowOver<Double> covarPop(Expression<? extends Number> expr1, Expression<? extends Number> expr2) {
          return new WindowOver<Double>(Double.class, SQLOps.COVARPOP, expr1, expr2);
      }
docs: CORR returns the coefficient of correlation of a set of number pairs.
  
  @param expr1 first arg
  @param expr2 second arg
  @return corr(expr1, expr2)