function getOpenDecoration(contents) {
    let decoration = ''
    for (const exception of exceptions) {
      const length = exception.length
      let index = 0
      while (contents.startsWith(exception, index)) {
        index += length
      }
      const exceptionLength = index
      if (decoration.length < exceptionLength) {
        decoration = contents.slice(0, exceptionLength)
      }
    }
    return decoration
  }
Get a open decoration string from comment contents.
@param {string} contents comment contents
@returns {string} decoration string