CR 118.9 - Some triggered abilities are written, “Whenever <a player> gains life, . . . .” Such abilities are treated as though they are written, “Whenever a source causes <a player> to gain life, . . . .” If a player gains 0 life, no life gain event has occurred, and these abilities won’t trigger.

This rule seems to imply that life gain is caused by a source.

A source of damage or source of a triggered ability is an object according to the CR glossary so it seems reasonable that a source that causes life gain is also an object.

Now from a prior ruling ( https://apps.magicjudges.org/forum/topic/11458/ ) it was determined that the spell Reverse Damage was not the cause of the life gain, rather it was the combat damage that caused it.

Would Firesong and Sunspeaker trigger off of the life gain of Reverse Damage?

The prior ruling appears to say no, yet 118.9 suggests that Reverse Damage is the source that causes the life gain.
Might we request a clarification?

Also, would it matter if the damage prevented by Reverse Damage was combat or non-combat damage?

Edited Todd Bussey (April 21, 2018 10:46:54 PM)