Error handling when returning undefined

[met4000]

Issue type:

• ➕ Feature request

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Short description:

Functions that return undefined (e.g. function f() { return undefined; }, or function f() {} ) give a rather verbose error as follows when apply0ed;

It would be desireable to either have: a dedicated error for a function returning undefined, rather than a translation error occuring (e.g. 'error: function returned undefined instead of a value'); or for an explicit 'undefined' singleton to be the output instead of erroring, allowing for logic such as equals(f(), undefined) on the InDy side.

[rubensworks]

Thanks for the suggestion!