Function interpolate 

pub fn interpolate(
    transaction: &Transaction,
) -> Result<InterpolationResult, InterpolationError>

Interpolate missing amounts in a transaction.

This function:

1. Identifies postings with missing amounts
2. For each currency, calculates the residual
3. Fills in the missing amount to balance

Rules

• At most one posting per currency can have a missing amount
• If a posting has a cost spec with a currency, that currency is used
• Otherwise, the posting gets the residual that makes the transaction balance

TLA+ Specification

Implements invariants from Interpolation.tla (post-#1030 redesign for N postings + multi-currency + cost-unknowns):

• AtMostOneUnknownPerCurrency: For each currency group, at most one posting may be “unknown” — either a missing amount (counts toward the units currency) or an empty cost spec like {} (counts toward the cost currency, since the cost-basis weight is unresolved until booking-pass lot matching). Returns MultipleMissing if violated.
• CompleteImpliesValidated: Interpolation only completes the transaction when the validation rule holds.

The spec models the structural validation rule, not the residual arithmetic that produces filled amounts — see Interpolation.tla’s header for the scope rationale.

See: spec/tla/Interpolation.tla

Example

let txn = Transaction::new(date, "Test")
    .with_synthesized_posting(Posting::new("Expenses:Food", Amount::new(dec!(50.00), "USD")))
    .with_synthesized_posting(Posting::auto("Assets:Cash"));

let result = interpolate(&txn)?;
// Assets:Cash now has -50.00 USD