What might "S/A-V-O" and "A/S-V-O" mean?

Question

In looking through Google Books's preview of A Grammar of Lao by N. J. Enfield, I came across these two terms, which I'm assuming are equivalent to one another (possibly one is even a typo) and some kind of variant of "SVO", representing the word order subject-object verb:

• S/A-V-O

  By unmarked I mean that the speakers report an impression that the S/A-V-O pattern is somehow basic in status. A consultant will likely supply the S/A-V-O pattern when asked to compose sentences of the variety The farmer killed the duckling, ...

• A/S-V-O

  With this in mind, consider the Lao A/S-V-O constituent order pattern, perhaps the closest to a pragmatically unmarked pattern:

  Departures from the A/S-V-O pattern are common. Movement, for example, may see a subject argument in a post-final position ...

Answer 1

The Abbreviations list of that book is viewable (if you search for "abbreviations"), and says "A = agent-like argument of transitive clause" and "S = single argument of intransitive clause".