Re: How do BEGIN/COMMIT/ABORT operate in a nested SPI query?

I am attempting to create a new language implementation. The language is Andl (andl.org), so the handler is plandl.
This is a question about executing SPI queries from inside plandl.

The documentation makes it clear that SPI allows nested queries; that in some instances it will be necessary to call
SPI_push()and SPI_pop(), but in others this will be handled automatically. Se
http://www.postgresql.org/docs/9.5/interactive/spi-spi-push.html.

It is an important design feature of plandl to allow nested queries.

My question is: where are the transaction boundaries if the inner/outer query do or do not contain BEGIN/ABORT/COMMIT?
Dothey nest, or does an inner COMMIT finish a transaction started by an outer BEGIN, or is it ignored? 

Regards
David M Bennett FACS

Andl - A New Database Language - andl.org