module PruningConfig implements ConfigSig {
  predicate isSource(Node source) {
    exists(MyState state | isSourceImpl(source, state))
  }

  predicate isSink(Node sink) { ... }
}

module PruningFlow = Global<PruningConfig>;

FlowState viableStateForSink(Node sink) {
  exists(PruningFlow::PathNode pSource, PruningFlow::PathNode pSink |
    PruningFlow::flowPath(pSource, pSink) and
    pSink.getNode() = sink and
    isSourceImpl(pSource.getNode(), result)
  )
}

module RealConfig implements StateConfigSig {
  class FlowState = MyState

  predicate isSource(Node source, FlowState state) { isSourceImpl(source, state) }

  predicate isSink(Node sink, FlowState state) {
    ... and state = viableStateForSink(sink) // <-- to prevent CP with all flow states.
  }

  predicate isBarrier(Node barrier, FlowState state) { ... }
}

module RealConfig implements StateConfigSig {
  predicate isSource(Node source, FlowState state) { ... }

  predicate isSink(Node sink) { ... }

  predicate isBarrier(Node barrier, FlowState state) { ... }
}