The Prompt Substitution Operator |||

The operator ||| is called the prompt substitution operator. It is used to create a substitute prompt. A substitute prompt is a prompt where the first prompt is queued for playback whenever the substitute prompt is used in a media context. If a failure occurs while attempting to queue the prompt then the substitute is queued instead. For example the main prompt could represent a TTS prompt which in cases where the system has not been installed or licensed with TTS support, one would want to fallback to a pre-recorded prompt. In this case, queuing a TTS prompt would fail and the substitute would be used instead. This operator is not associative.

					SubstituteExpression:
									PromptExpression ||| PromptExpression