What is sset_trace?

[galenseilis]

On the README I see this example:

import ipdb
ipdb.sset_trace()
ipdb.spm()

from ipdb import slaunch_ipdb_on_exception
with slaunch_ipdb_on_exception():
    ...

At first I thought that sset_trace, but when I ran it in an interactive Python session I found that it appeared to start a trace... but I am not clear about why that worked or why it exists as an alternative to set_trace.

What is sset_trace? How does it differ from set_trace?