Perhaps we can get at the same issue in a slightly different way with slightly different wording. If we added to the existing wording, put in a comma and said “should any of the above be unavailable, a formally substituted member be allowed as a replacement”, that resolves the issue that was just raised.
Evidence of meeting #1 for Access to Information, Privacy and Ethics in the 40th Parliament, 2nd session. (The original version is on Parliament’s site, as are the minutes.) The winning word was amendment.
A recording is available from Parliament.