Function credit_id 

pub(crate) fn credit_id(spelling: &str) -> PyResult<CreditCurveId>

A construction refusal from a product-level core error: the input never reached a valuation, so it is the caller’s ValueError, as a bad currency code is. A credit curve identity from its printed spelling — ACME/SeniorUnSec/XR14/USD or CDX.NA.IG.45v1. A refusal is the core’s, raised as ValueError as every malformed caller input is.