Value-equals test required, constraint on grandchild (a co-constraint use case)
A given element does not occur (is no longer optional but forbidden), if the value of another element -- a left-sibling of its parent, in the example here -- is equal to a given constant.
Cf. Value equality - and null/non-null, which has a similar scenario but does not specify the relation of the controlling element to the controlled element (here: aunt to niece). Cf. also Value equality - and children of following sibling, which appears to be essentially the same as this use case.
source: San Coulibaly 
Other use cases: Co-constraint Use Cases
<moi> <A>30</A> <B>40</B> <C> <D>Hello</D> <E>20</E> </C> </moi>
Now, I want to generate, if A=10, then element D in C does not appear!
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