I noticed something recently about the 16 steps of anapana. There's an asymmetry when you compare the last two items of vedananupassana and the last two items of kayanupassana.
The last two of vedananupassana:
8. passambhayam cittasankharam
the last two of kayanupassana:
3. sabba kaya patisamvedi
4. passambhayam [u]kaya sankharam[/u]
So #7 and #8 both use citta sankhara, whereas #3 and #4 there's a change - one is "sabba kaya", and the other is "kaya sankahra". This is why it seems really odd that commentaries interpret both "sabba kaya" and "kaya sankhara" to mean "the breath". If the Buddha wanted to refer to the same concept, why would he change the terminology and risk confusing novice monks when in steps 7 and 8 he's consistent?
I always felt that the argument of the commentaries that "the breath is a body among bodies" to justify "sabba kaya" as breath ( MN 118 ) is like looking at the wrong end of the telescope. To me the straightforward analysis is that the Buddha is explaining why "the breath" is classified under "KAYA"nupassana, body contemplation. Just as in the next tetrad, he explains that "in-&-out breaths — is classed as a feeling among feelings". He's saying that to explain how one can stay with the breath as the object of meditation and still be classified as "vedananupassana", contemplation of feelings.
from mn 118
(referring to kayanupassana)
I tell you, monks, that this — the in-&-out breath — is classed as a body among bodies, which is why the monk on that occasion remains focused on the body in & of itself — ardent, alert, & mindful — putting aside greed & distress with reference to the world.
(referring to cittanupassana)
I tell you, monks, that this — careful attention to in-&-out breaths — is classed as a feeling among feelings, which is why the monk on that occasion remains focused on feelings in & of themselves — ardent, alert, & mindful — putting aside greed & distress with reference to the world.
If you take the commentary argument to its conclusion, you would also have to say that for step 7 and step 8 of anapana, one is not supposed to look at cittasankhara as "mental formation", but instead as the physical breath. And in step 8, one is not to calm citta sankhara but to calm the physical breath instead. But then you would wonder, why would the Buddha be redundant and have step 3, 4, 7,8 all refer to physical breath?