code: function createProducer(tapVersion) {
    var producers = {
      '12': new TAP12Producer(),
      '13': new TAP13Producer()
    };
    var producer = producers[tapVersion];
  
    if (!producer) {
      throw new Error(
        'invalid or unsupported TAP version: ' + JSON.stringify(tapVersion)
      );
    }
  
    return producer;
  }
docs: Returns a `tapVersion`-appropriate TAP producer instance, if possible.
  
  @private
  @param {string} tapVersion - Version of TAP specification to produce.
  @returns {TAPProducer} specification-appropriate instance
  @throws {Error} if specification version has no associated producer.