Under what condition is a mule's leftover water classified as doubtful according to the footnotes of Page 12?

Classifications of Water: Impure & Doubtful Waters

• Only if the mule is larger than a sheep.
• Only if the mother of the mule was a female donkey (atan), not a mare.
• Only if the mule drank from a running river.
• It is never classified as doubtful.