* e.g.:
 * ```
 * reduce({a: [b, c], b: [e1], c: [d], d: [e2]}) => {a: [e1, e2], b: [e1], c: [e2], d: [e2]}
 * ```
 */
internal
fun <V> reduceGraph(graph: Map<V, List<V>>): Map<V, Set<V>> {

    val result = mutableMapOf<V, Set<V>>()

    val reducing = hashSetOf<V>()

    fun reduce(node: V, refs: Iterable<V>): Set<V> {

        result[node]?.let {

import org.hamcrest.CoreMatchers.equalTo

import org.junit.Test


class ReduceGraphTest {

    @Test
    fun `only external node references are left in the graph`() {

        assertThat(
            reduceGraph(
                mapOf(
                    "a" to listOf("b", "c"),
                    "b" to listOf("e1"),
                    "c" to listOf("d"),
                    "d" to listOf("b", "e2")
                )

            it.scriptPlugin.id
        }

        val pluginGraph = plugins.associate {
            it.id to pluginsAppliedBy(it, scriptPluginsById)
        }

        return reduceGraph(pluginGraph).asSequence().mapNotNull { (id, plugins) ->
            scriptPluginsById[id]?.copy(appliedPlugins = plugins.toList())
        }
    }

    private