Error: The given system is mixed-determined. [index > 3] is very confusing

[Francesco Casella]

Sometimes the back-end stops with this error:

Error: The given system is mixed-determined.   [index > 3]
Please checkout the option "--maxMixedDeterminedIndex".
Error: No system for the symbolic initialization was generated

see, e.g., ​Modelica.Electrical.Analog.Examples.ResonanceCircuits.

The message is quite obscure, for three main reasons:

• it doesn't make it explicit that the issue is only with the 'initialization problem'. I know it pops up while the backend is processing the initialization problem, but it's not obvious anyway.
• it refers to the term 'index' which has a well-defined 'completely different' meaning in the context of DAE modelling, and is very, very confusing.
• it doesn't really help understanding what to do, but mostly just scares off users.

I myself only recently understood that this somewhat means the initialization problem is under- and over-determined at the same time (though I'm still not 100% sure what this message really means). If that is the case, I would write that explicitly.

I would strongly suggest to improve the situation by changing the message, so that people do not fall into one of the three semantic traps I listed above. I'm available to discuss the details of the message in this ticket.