     /**
         An `AudioStreamBasicDescription` that describes the current data format for this audio stream.
 
-        - Returns: A `AudioStreamBasicDescription`
+        - Returns: *(optional)* A `AudioStreamBasicDescription`
      */
     public lazy var physicalFormat: AudioStreamBasicDescription? = {
         guard let direction = self.direction else {



    /**
     An array of all the available physical formats for this audio stream.

     - Returns: An array of `AudioStreamRangedDescription`
     */
    public lazy var availablePhysicalFormats: [AudioStreamRangedDescription]? = {
        guard let direction = self.direction else {

        **Discussion:** By default, non-mixable streams are returned, however, these can be filtered 
        out by setting `includeNonMixable` to `false`.

        - Returns: *(optional)* An array of `AudioStreamBasicDescription`
     */
    public final func availablePhysicalFormatsMatchingCurrentNominalSampleRate(includeNonMixable: Bool = true) -> [AudioStreamBasicDescription]? {
        guard let physicalFormats = availablePhysicalFormats,
