Fix minimizer

src/main/kotlin/Main.kt

@@ -37,8 +37,9 @@ fun main() {
     // Minimize transactions:
     for ((user, transactions) in payments.iterator()) {
         for ((target, pay) in transactions.iterator()) {
-            val payback = payments[target]?.firstNotNullOf { (other, euros) ->
-                assert(user == other)
+            val payback = payments[target]
+                ?.filter { (other, _) -> other == user }
+                ?.firstNotNullOfOrNull { (_, euros) ->
                     euros
                 } ?: continue
             val common = min(pay, payback)
@@ -51,6 +52,7 @@ fun main() {
             payments[target] = payments[target]!!.also {
                 it[user] = newPayback
             }
+            println("Minimized $user -> $target for $common")
         }
     }