/**
 * Narrow a REST string onto an entity enum: the projection-side policy for
 * Google's output enums (EXTENDING.md, "Enum policy"). Unknown and `*_UNSPECIFIED` values drop
 * to `undefined` (the field goes absent) rather than passing through untyped
 * or being coerced to a wrong-but-valid value.
 */
export declare function narrow<T extends string>(value: string | null | undefined, allowed: readonly T[]): T | undefined;
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