Commentary Raahulovaadasutta, no 6.
Sutta:
aanaapaanasati Raahula bhaavitaa bahuliikataa mahapphalaa hoti
mahaanisa.msaa ti.

The development of mindfulness on in and out breathing, Rahula, is of
great fruit and profit."

Commentary:
mahapphalaa hotiiti kiivamahapphalaa hoti? idha bhikkhu aanaapaanassati.m
anuyutto ekaasane nisinnova sabbaasave khepetvaa arahatta.m paapu.naati,
tathaa asakkonto mara.nakaale samasiisii hoti, tathaa asakkonto devaloke
nibbattitvaa dhammakathikadevaputtassa dhamma.m sutvaa arahatta.m
paapu.naati, tato viraddho anuppanne buddhuppaade paccekabodhi.m
sacchikaroti, ta.m asacchikaronto buddhaana.m sammukhiibhaave
baahiyattheraadayo viya khippaabhi~n~no hoti, eva.m mahapphalaa.
mahaanisa.msaati tasseva vevacana.m.

Mahapphala hotii: it is very fruitful. Why is it very fruitful? Here, the
monk who is intent on mindfulness of breathing eradicates all defilements
and reaches arahatship in one session. If he cannot do this (asakkonto),
there will be at the end of life the extinction of all
defilements(samasiisi, who attains two ends simultaneously). If he cannot do
this, he will be reborn in a deva plane, and when he has heard Dhamma from a
deity (devaputto, son of a deva) who is a speaker of Dhamma (dhammakathika),
he will attain arahatship. If this failed (viraddho) because the appearance
of a Buddha had not taken place (anuppanne Buddhuppaade), he will realize
the awakening of a solitary Buddha (Paccekabodhi.m). If he cannot realize
this, he can, being in the presence (sammuukhiibhaave) of the Buddhas,
realize the truth quickly (kippaabhi~n~no), such as the Elder Baahiya and
others. The words mahapphalaa and mahaanisa.msaa, very fruitful, very
beneficial, are synonyms.

vuttampi ceta.m --
``aanaapaanassatii yassa, paripu.n.naa subhaavitaa.
anupubba.m paricitaa, yathaa buddhena desitaa.
soma.m loka.m pabhaaseti, abbhaa muttova candimaa''ti.. (theragaa0 548;
pa.ti0
ma0 1.1.60) --

The Buddha spoke as follows:
³The person who fully develops Mindfulness of Breathing,
Who gradually accumulates it as taught by the Buddha,
He brightens the world as the moon that is free from clouds.²

ima.m mahapphalata.m sampassamaano thero saddhivihaarika.m tattha niyojeti.
iti bhagavaa ruupakamma.t.thaana.m, thero aanaapaanassatinti ubhopi
kamma.t.thaana.m aacikkhitvaa gataa, raahulabhaddo vihaareyeva ohiino.

The Elder knew that Mindfulness of breathing was of great fruit and
therefore he urged his pupil to develop it. Thus, the Blessed One taught the
meditation subject of physical phenomena (rúpa), and the Elder taught
Mindfulness of Breathing; when both of them had explained these subjects,
they went away, and Lucky Rahula was left behind in the dwelling place.

*****
Nina