What I can figure out:
mustSpendAtLeast is basically just a wrapper around MustSpendAtLeast. This gets processed here, by adding a required value to this thing. These values get turned into outputs in addMissingValueSpent.
With all this, I still don't understand how this constraint is supposed to be used.
edit:
The root of my inability to figure this out was that I was looking in the wrong place for the information. Ledger.Constraints.OnChain is the place to go to figure out what all the constraints actually mean. In the case of mustSpendAtLeast
, it simply means that all the values of all the inputs to the transaction must add up to at least that amount.