A Vastly Underrated Tense

Son: I'm hungry. I hardly ate any dinner. And I had no dessert either.
Me: Then eat something.
Son: Okay. And that thing was mostly true.
Me: What thing?
Son: About dinner and dessert.
Me: ... Well, I expect that most things you tell me will be at least "mostly true."
Son: Then: so it shall be!

***

Me: Could you clear the table?
Son: Never fear, Mother! It shall be done!