Function collateral_ladder_indices 

fn collateral_ladder_indices(
    rungs: &[Rung],
) -> Result<(Option<&str>, Option<&str>), CalibrationError>

The pay and receive projection indices a collateral ladder’s rungs declare, read off any rung that names one, and the check that every rung solves the same collateral key.

Errors

CalibrationError::MixedCollateral naming two keys; a rung that declares no foreign discount at all is refused as malformed.