Lifedebt Question

[Bee chase]

No: HPFGUIDX 142354

Luckdragon:
I know that various lifedebts have been discussed in the past...

ie) Snapes lifedebt to James
    Wormtails lifedebt to Harry
    Snapes lifedebt to Dumbledore
...and I did a search, but did not find anything about DD owing a 
lifedebt to Snape. Did I miss it?

I'm wondering if Snape saving DD after the ring incident could have 
created this particular lifedebt. If DD owed Snape his life and Snape 
would have died by not fullfilling his Unbreakable vow, it could 
explain why DD begged Snape to kill him.