diff --git a/src/main/java/reactivestreams/ColumnChecker.java b/src/main/java/reactivestreams/ColumnChecker.java
index 4df362b..297be2b 100644
--- a/src/main/java/reactivestreams/ColumnChecker.java
+++ b/src/main/java/reactivestreams/ColumnChecker.java
@@ -13,4 +13,8 @@ public class ColumnChecker extends DigitBlockChecker {
 		return currentColumn == column;
 	}
 
+	@Override
+	public String toString() {
+		return getClass().getSimpleName() + " " + column;
+	}
 }
diff --git a/src/main/java/reactivestreams/DigitBlockChecker.java b/src/main/java/reactivestreams/DigitBlockChecker.java
index 655304f..77854d3 100644
--- a/src/main/java/reactivestreams/DigitBlockChecker.java
+++ b/src/main/java/reactivestreams/DigitBlockChecker.java
@@ -35,11 +35,12 @@ public abstract class DigitBlockChecker extends SubmissionPublisher<Boolean> imp
 			if (digitsProcessed.contains(digit)) {
 				submit(false);
 				subscription.cancel();
-			}
-			digitsProcessed.add(digit);
-			if (digitsProcessed.size() == 9) {
-				submit(true);
-				subscription.cancel();
+			} else {
+				digitsProcessed.add(digit);
+				if (digitsProcessed.size() == 9) {
+					submit(true);
+					subscription.cancel();
+				}
 			}
 		}
 	}
@@ -54,7 +55,7 @@ public abstract class DigitBlockChecker extends SubmissionPublisher<Boolean> imp
 
 	@Override
 	public void onComplete() {
-		close();
+		// not called
 	}
 
 }
diff --git a/src/main/java/reactivestreams/RowChecker.java b/src/main/java/reactivestreams/RowChecker.java
index 7212ef6..f4927c3 100644
--- a/src/main/java/reactivestreams/RowChecker.java
+++ b/src/main/java/reactivestreams/RowChecker.java
@@ -13,4 +13,8 @@ public class RowChecker extends DigitBlockChecker {
 		return currentRow == row;
 	}
 
+	@Override
+	public String toString() {
+		return getClass().getSimpleName() + " " + row;
+	}
 }
diff --git a/src/main/java/reactivestreams/SquareChecker.java b/src/main/java/reactivestreams/SquareChecker.java
index 4580e25..20bb299 100644
--- a/src/main/java/reactivestreams/SquareChecker.java
+++ b/src/main/java/reactivestreams/SquareChecker.java
@@ -27,4 +27,9 @@ public class SquareChecker extends DigitBlockChecker {
 		return currentColumn >= fromColumn && currentColumn <= toColumn;
 	}
 
+	@Override
+	public String toString() {
+		return getClass().getSimpleName() + " " + squareRow + "/" + squareColumn;
+	}
+
 }
diff --git a/src/main/java/reactivestreams/SudokuVerifier.java b/src/main/java/reactivestreams/SudokuVerifier.java
index 8f2354b..6b3e384 100644
--- a/src/main/java/reactivestreams/SudokuVerifier.java
+++ b/src/main/java/reactivestreams/SudokuVerifier.java
@@ -30,13 +30,12 @@ public class SudokuVerifier implements Flow.Subscriber<Boolean> {
 	}
 
 	public boolean isSolved() {
+		int subscriberCount = digitPublisher.getSubscribers().size();
 		// Make publisher emit each digit
 		digitStream.forEach(digitPublisher::submit);
 		digitPublisher.close();
 		// Wait for checking subscribers to finish
-		int subscriberCount = digitPublisher.getSubscribers().size();
-		while (true) {
-			if (!(finished.get() < subscriberCount && solved.get())) break;
+		while ((finished.get() < subscriberCount) && solved.get()) {
 			justAMoment();
 		}
 		return solved.get();
@@ -74,17 +73,17 @@ public class SudokuVerifier implements Flow.Subscriber<Boolean> {
 		if (! solved) {
 			this.solved.compareAndSet(true, false);
 		}
+		finished.incrementAndGet();
 	}
 
 	@Override
 	public void onError(Throwable throwable) {
 		System.err.println(throwable.fillInStackTrace());
 		finished.incrementAndGet();
-		System.exit(1);
 	}
 
 	@Override
 	public void onComplete() {
-		finished.incrementAndGet();
+		// not called
 	}
 }