public Map<String, String> getOperationInputBindings(ToShapeId resource, ToShapeId operation) {
        return Optional.ofNullable(inputBindings.get(resource.toShapeId()))
                .flatMap(resourceMap -> Optional.ofNullable(resourceMap.get(operation.toShapeId())))
                .map(Collections::unmodifiableMap)
                .orElseGet(Collections::emptyMap);
    }
Gets a map of identifier names to input member names that provide a
value for that identifier.

@param resource Shape ID of a resource.
@param operation Shape ID of an operation.
@return Returns the identifier bindings map or an empty map if the
 binding is invalid or cannot be found.
smithy-model/src/main/java/software/amazon/smithy/model/knowledge/IdentifierBindingIndex.java