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

module.exports = checkStrong

function checkStrong(context) {
  var marker = context.options.strong || '*'

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

  return marker
}