3 statement switches

[oxinabox]

This is a minor annoyance for maintenance. Which complicates things when you realize that the IR has encountered something unexpected, or is doing something wrong, you need to look in 3 places.

These are each basically big switch statements, that decide how to AD particular IR statments

• Reverse
• Forward
• Forward-Demand

It seems like we should only have 2.
That we can perhaps collapse forward as a special case of Forward-Demand where you demand everything or some such.

But they are not trivial to collapse.