Crosswalk a US medical code or drug across systems and within a hierarchy. Hierarchy directions: `parents` and `children` walk a code's prefix hierarchy one level per call — immediate parent/children only (depth-1); call iteratively for the full ancestor or descendant path (ICD-10-CM/HCPCS; ICD-10-PCS codes have no prefix parent, and RxNorm concepts no code hierarchy). A resolvable source with no edge in the requested direction is a successful empty result with a notice, not an error. A source code string that also exists in another bundled system carries `alsoInSystems` naming it, since only the resolved system's hierarchy was walked. Drug directions (RxNorm): `name_to_rxcui` (drug name → RXCUI), `ndc_to_rxcui` and `rxcui_to_ndc` (NDC ↔ RXCUI; NDCs accepted hyphenated in an FDA segment configuration — 4-4-2, 5-3-2, 5-4-1, or the 11-digit 5-4-2 — or as bare 10/11 digits; `ndc_to_rxcui` names the product it decoded to), `rxcui_to_ingredients` and `rxcui_to_brands` (RXCUI → ingredient/brand RXCUIs, each with the target's RxNorm name and its `conceptType` — read that before counting a combination product's ingredients). Drug-class directions (RxClass): `rxcui_to_classes` (RXCUI → its classes: pharmacologic class, mechanism of action, physiologic effect, pharmacokinetics, therapeutic category, chemical structure, the diseases it may treat, prevent, diagnose, or induce or is contraindicated with, VA class, DEA controlled-substance schedule, CDC vaccine code; a drug product also carries its ingredients' classes, naming the ingredient in `via`, while DEA schedules are recorded only on drug products and VA classes almost only there, so map a product for those) and `class_to_rxcuis` (class ID → its direct member RXCUIs, each with its RxNorm name and `conceptType`). Each class hit carries `classType`, `source` (the RxClass source asserting it), and `relation` — a `ci_` relation is a contraindication, not an indication; narrow either direction with `classType`. Every result carries `source` provenance (which system or edge answered) so a chained call (e.g. into openfda with a resolved NDC) uses the right identifier. The `children`, `name_to_rxcui`, `rxcui_to_ndc`, `rxcui_to_classes`, and `class_to_rxcuis` directions can return large sets and paginate: a `nextCursor` in the response is passed back as `cursor` (with an optional `limit` page size) to walk the full set. A field the direction does not use is rejected with `field_not_applicable`: `limit` and `cursor` on the point directions, `classType` outside the class directions, and a `system` other than RXNORM on the drug and class directions.
