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:

• AtMostOneNull: At most one posting per currency can have a missing amount (returns MultipleMissing error if violated)
• CompleteImpliesBalanced: After interpolation, sum(postings) = 0 for each currency
• HasNullAccurate: filled_indices contains exactly the indices of postings that were originally missing amounts

See: spec/tla/Interpolation.tla

Example

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

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