results merged into the `assignment`. /// /// - Parameters: /// - constraints: The input list of constraints to solve. /// - assignment: The assignment to merge the result into. /// - allConstraints: An additional set of constraints on the viable solutions. /// - allExclusions: A set of package assignments to exclude from consideration. /// - Returns: A sequence of all valid satisfying assignment, in order of preference. private func merge( constraints: [Constraint], into assignment: AssignmentSet, subjectTo allConstraints: ConstraintSet, excluding allExclusions: [Identifier: Set<Version>] ) -> AnySequence<AssignmentSet> { var allConstraints = allConstraints if enablePrefetching { // Ask all of these containers upfront to do async cloning. for constraint in constraints { provider.getContainer(for: constraint.identifier) { _ in } } }