This is a non-trivial refactor that slightly changes the semantics
of the internal definition lists.
Whereas previously only individual list items would trigger the exception,
now the error is promoted to the whole list.
This is mostly ok, because we compute the value, it is wrong to ignore a definition.
However, we don't always compute the value. For instance `readOnly`
only needs to count definitions. That won't be possible anymore when
the error is raised for one of the items. As a consequence, an error
will be raised for the errant definition instead of the number of
definitions.
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