/**
 * Vary Params Decoding
 *
 * This module is shared between server and client.
 */
export type VaryParams = Set<string>;
/**
 * Vary params are serialized into the Flight stream as an
 * `AsyncIterable<string>` that yields each accessed param name exactly once
 * (the server dedupes before emitting). Because each access is flushed into the
 * stream as it happens, there's no step at the end of the render that has to
 * run for the client to read anything. If a prerender is aborted by sync I/O,
 * the params yielded before the abort are already in the stream, and they're
 * exactly the params the partial response actually depends on.
 *
 * Root params are NOT included in a segment's own iterable. They're emitted
 * once at the top level of the response (as a separate iterable) and unioned in
 * by `readVaryParams`, because root params can be accessed at any point during
 * the render — folding them into every segment would otherwise require a merge
 * once the whole render is complete.
 */
export type VaryParamsIterable = AsyncIterable<string>;
/**
 * Reads a segment's (or the head's) vary params, unioning in the response-level
 * root params.
 *
 * Root params are emitted once at the top level rather than folded into every
 * segment by the server, so every read recombines them here — building the
 * merge into the read means a caller can't forget it, and it's done in a single
 * pass with no intermediate set.
 *
 * Returns null ("unknown", key on all params) unless BOTH iterables are
 * present. A null/absent `iterable` means the segment's own tracking wasn't
 * enabled (e.g. not a prerender). A null/absent `rootIterable` means root
 * params weren't tracked — and since a segment's own iterable never includes
 * root params (those are accessed in layouts above it), narrowing on the
 * segment set alone would wrongly assume no root params were accessed. In
 * either case we stay conservative.
 *
 * When both are present each is authoritative even when it drains to the empty
 * set — a tracked segment that read no params, with no root params accessed,
 * can be shared across all param values.
 */
export declare function readVaryParams(iterable: VaryParamsIterable | null | undefined, rootIterable: VaryParamsIterable | null | undefined): VaryParams | null;