/**
 * Asserts that a value is a number.
 *
 * Throws a TypeError if the input is not a number. Uses TypeScript's type assertion to narrow the type.
 *
 * @param x The value to check
 * @throws If x is not a number
 *
 * @example Valid number
 * ```ts
 * import { assertThrows } from "jsr:@std/assert";
 *
 * assertnumber(5); // No error
 *
 * ```
 *
 * @example Invalid input throws error
 * ```ts
 * import { assertThrows } from "jsr:@std/assert";
 *
 * assertThrows(() => assertnumber("5"), TypeError, "Expected number, got string");
 *
 * ```
 *
 * @example NaN is a valid number
 * ```ts
 * import { assertThrows } from "jsr:@std/assert";
 *
 * assertnumber(NaN); // No error
 * ```
 */
export default function assertnumber(x: unknown): asserts x is number;
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