in src/riff-raff-yaml-file/index.ts [189:195]
private validateAllRegionsAreResolved(): void {
const unresolved = this.allRegions.filter((region) => Token.isUnresolved(region));
if (unresolved.length !== 0) {
throw new Error(`Unable to produce a working riff-raff.yaml file; all stacks must have an explicit region set`);
}
}