Dumbledore & Snape's Unbreakable Vow: who's the Bonder / Witness?

[Eddie]

No: HPFGUIDX 160078

OK, supposing for a moment that Dumbledore and Snape made an
unbreakable vow -- which is, in this hypothetical scenario, why
Dumbledore trusts Snape -- then who was their bonder / witness?

Thoughts:
* Someone from the Order of the Phoenix?
* McGonagall?
* Aberforth?
* Flitwick?

See http://en.wikipedia.org/wiki/Unbreakable_Vow for background.

Eddie