Clarify impure in systems of equations.

[HansOlsson]

No description provided.

[henrikt-ma]

The main issue which I still find unresolved is that we both have

1. It is an error if an impure function call is part of a systems of equations (including linear systems), even if called in agreement with the restrictions above.

and

2. …if the function call of an impure function is part of an algebraic loop. … This can occur…

Could we avoid this contradiction by a more careful formulation of 1, something like this:

Inside systems of equations (including linear systems), an impure function call is only allowed in the deprecated case of an external function neither marked pure or impure, or when pure(…) is used to bypass purity checking. The system of equations shall then be solved as if all occurring functions were pure, but the side-effect semantics of the impure function calls are undefined. Specifically, the number of times an impure function is called when solving the equations is undefined.

[HansOlsson]

@henrikt-ma this should now handle the comments.

[HansOlsson]

@henrikt-ma is it ok now?

[henrikt-ma]

Just some wording and formatting remaining.

[HansOlsson]

Just some wording and formatting remaining.

I think that has now been handled.

[henrikt-ma]

Can't see anything that should prevent merging.