Add Constraint AASd-133 in SpecificAssetId

[zrgt]

Check if external_subject_id is ExternalReference

fixes #145

[jkhsjdhjs]

This constraint was already enforced by the typechecker, as we declare the type as ExternalReference. Not sure if we should implement this exception anyway.

[s-heppner]

Also, you probably mean #145 don't you?

[s-heppner]

This constraint was already enforced by the typechecker, as we declare the type as ExternalReference. Not sure if we should implement this exception anyway.

I think we should check it explicitly, since the type checker only analyzes the code statically, whereas this implementation also ensures correctness during runtime.

[s-heppner]

Looks good to me. I'll rebase if you say it's ready.

[zrgt]

@s-heppner it's ready

[jkhsjdhjs]

I think we should check it explicitly, since the type checker only analyzes the code statically, whereas this implementation also ensures correctness during runtime.

We should stay consistent though. We don't verify any other types because the typechecker does it for us. Everything that can be verified by the typechecker is currently only verified by the typechecker and not at runtime. If a variable is declared as a certain type, we always assume that the variable is of the declared type. Why shouldn't we do it in this case?

[s-heppner]

We decided not to implement the check for reasons of consistency. However, we leave the documentation that this constraint exists.