Convert stderrLevels into an Object for faster key-lookup times than an Array.  For backwards compatibility, stderrLevels defaults to ['error', 'debug'] or ['error'] depending on whether options.debugStdout is true.
function setStderrLevels (levels, debugStdout) {
    var defaultMsg = 'Cannot have non-string elements in stderrLevels Array';
    if (debugStdout) {
      if (levels) {
        //
        // Don't allow setting both debugStdout and stderrLevels together,
        // since this could cause behaviour a programmer might not expect.
        //
        throw new Error('Cannot set debugStdout and stderrLevels together');
      }

      return common.stringArrayToSet(['error'], defaultMsg);
    }

    if (!levels) {
      return common.stringArrayToSet(['error', 'debug'], defaultMsg);
    } else if (!(Array.isArray(levels))) {
      throw new Error('Cannot set stderrLevels to type other than Array');
    }

    return common.stringArrayToSet(levels, defaultMsg);
  }