SPI isolation changes

Hi,

When I read the documents and coding of SPI, [1]
I found that the following the SPI_start_transaction does not support
transaciton_mode(ISOLATION LEVEL, READ WRITE/READ ONLY) like BEGIN 
command. [2]
Is there a reason for this?

I would like to be able to set transaciton_mode in 
SPI_start_transaction.
What do you think?

[1]
https://www.postgresql.org/docs/devel/spi-spi-start-transaction.html

[2]
https://www.postgresql.org/docs/devel/sql-begin.html

Regards,
-- 
Seino Yuki
NTT DATA CORPORATION