function parse(options) {
  const settings = options || {}
  const constructs = /** @type {FullNormalizedExtension} */ (
    combineExtensions([defaultConstructs, ...(settings.extensions || [])])
  )

  /** @type {ParseContext} */
  const parser = {
    constructs,
    content: create(content),
    defined: [],
    document: create(document),
    flow: create(flow),
    lazy: {},
    string: create(string),
    text: create(text)
  }

  return parser

  /**
   * @param {InitialConstruct} initial
   *   Construct to start with.
   * @returns {Create}
   *   Create a tokenizer.
   */
  function create(initial) {
    return creator
    /** @type {Create} */
    function creator(from) {
      return createTokenizer(parser, initial, from)
    }
  }
}
@param {ParseOptions | null | undefined} [options]
  Configuration (optional).
@returns {ParseContext}
  Parser.