Function explain 

pub fn explain(
    book: &Book,
    before: &MarketContext,
    after: &MarketContext,
    order: &[RiskAxis],
) -> Result<ExplainReport, PnlError>

The step-revaluation explain of book from before to after: first the prints after holds that before does not, published onto before as the ExplainLeg::Resets leg (present only when there are any), then one axis group at a time in order: at its step an axis takes after’s rate, repo, security-spread, credit or inflation curves (shock transforms included) and after’s value of every parameter on it. Axes absent from order, and inputs on no axis, move together in the final leg, so the last step is after.

Errors

The layout guard, a valuation, or PnlError::MoveOutsideParameters when the last step does not value bit for bit as after — the markets differ by something no step moves.