Transaction manipulation commands in user defined c functions?

I've been searching for a way to do transaction blocks and savepoints within a user defined c function. Apparently the docs state that if an SPI function throws an error, control goes out of the function and the current transaction is rolled back, yet it hints that you can do subtransactions within a c function.

a. Is that possible?
b. I couldn't find any doc or reference to such a feature.

Has anyone got an example doing this? Can you please just point me to the right direction?

Michael.
#avg_ls_inline_popup { position:absolute; z-index:9999; padding: 0px 0px; margin-left: 0px; margin-top: 0px; width: 240px; overflow: hidden; word-wrap: break-word; color: black; font-size: 10px; text-align: left; line-height: 13px;}