/**
 * Headings are single line only (don't allow multiline).
 * - `#` 1-6 hashes, then one or more spaces, then the title.
 * - `#` must be the first character on the line.
 * - Markdown's underline syntax is not supported (for simplification).
 */
export declare const HEADING_RULE: import("../util/rule.js").MarkupRule;