Coming from a point of view of TDD, designing `_CIM_extract`, how do I know that it provides the correct output and that is only by coincidence that Constrain.extract returns the result I expect.  I would write a unit test to describe the behaviour/logic I'm coding, not the result from a chain of events.  I would then write one/two integration tests for this chain.

Illustration:
`(IN) FunctionA (OUT)` - `(IN) FunctionB (OUT)`

This test specifies IN of FunctionA and ensures the result OUT of FunctionB.  Is this coincidence? what was FunctionB called with?

I would mock the Constraint.extract method, and do an `assert_Called_With`, thus unit testing `(IN) FunctionA (OUT)`

I think my argument is prob. out of scope for this PR.