The test rank of a soluble product of free Abelian groups

We consider the variety $\mathbb A^l$ of all soluble groups of derived length at most $l$, $l\geqslant2$. Suppose that a finitely generated group $G$ is a free product in the variety $\mathbb A^l$ of Abelian torsion-free groups. It is proved that the test rank of $G$ is one less than the number of factors. A test set of elements is written out explicitly.
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