Chen's few-subpowers expressive-rate conjecture

Let $\Gamma$ be a constraint language, and let $r(n)$ be the logarithm of the number of distinct $n$-variable relations definable by primitive positive formulas over $\Gamma$. Chen's expressive-rate conjecture. The function $r(n)$ always either grows as a polynomial or as an exponential function. In the polynomial-growth case, the class of definable relations is efficiently learnable and the associated CSP can be solved in polynomial time. The surrounding text says that this conjecture was resolved through the theory of algebras with few subpowers.