node_modules/mdast-util-to-markdown/lib/util/check-rule.js

module.exports = checkRule

function checkRule(context) {
  var marker = context.options.rule || '*'

  if (marker !== '*' && marker !== '-' && marker !== '_') {
    throw new Error(
      'Cannot serialize rules with `' +
        marker +
        '` for `options.rule`, expected `*`, `-`, or `_`'
    )
  }

  return marker
}