/**
 * The `shadow` prop (`box-shadow` syntax) as the silhouette's `filter`, or
 * `undefined` to keep the stylesheet default.
 *
 * `drop-shadow()` follows the path's alpha, so the shadow hugs every bud,
 * neck and drop; its blur radius means what box-shadow's does, and it is the
 * same conversion TxLiquid applies to its blurred outer layers. It has no
 * spread and no inset, so those layers are skipped. A `var()` layer is
 * passed through whole: `var(--tx-elevation-4)` works because each elevation
 * token is a single `x y blur colour` layer.
 */
export declare function shadowFilter(shadow: string | null | undefined): string | undefined;