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New lib/datalog/aggregates.sx: (count R V Goal), (sum R V Goal), (min R V Goal), (max R V Goal). dl-eval-aggregate runs dl-find-bindings on the goal under the outer subst, collects distinct values of V, applies the operator, binds R. Empty input: count/sum return 0; min/max produce no binding (rule fails). Group-by emerges naturally from outer-subst substitution into the goal — `popular(P) :- post(P), count(N, U, liked(U, P)), >=(N, 3).` counts per-post. Stratifier extended: dl-aggregate-dep-edge contributes a negation-like edge so the aggregate's goal relation is fully derived before the aggregate fires (non-monotonicity respected). Safety relaxed for aggregates: goal-internal vars are existentials, only the result var becomes bound.