diff --git a/pom.xml b/pom.xml
index 4bdbe4f..ee0400b 100644
--- a/pom.xml
+++ b/pom.xml
@@ -5,7 +5,7 @@
 
     <groupId>com.google.pdsl</groupId>
     <artifactId>pdsl</artifactId>
-    <version>1.8.2</version>
+    <version>1.8.3-SNAPSHOT</version>
 
     <name>pdsl</name>
     <url>http://www.github.com/google/polymorphicDSL</url>
diff --git a/src/main/java/com/pdsl/executors/DefaultPolymorphicDslTestExecutor.java b/src/main/java/com/pdsl/executors/DefaultPolymorphicDslTestExecutor.java
index f5aab1b..1e96acc 100644
--- a/src/main/java/com/pdsl/executors/DefaultPolymorphicDslTestExecutor.java
+++ b/src/main/java/com/pdsl/executors/DefaultPolymorphicDslTestExecutor.java
@@ -1,6 +1,5 @@
 package com.pdsl.executors;
 
-import com.pdsl.logging.AnsiTerminalColorHelper;
 import com.pdsl.logging.PdslThreadSafeOutputStream;
 import com.pdsl.reports.DefaultTestResult;
 import com.pdsl.reports.MetadataTestRunResults;
@@ -9,25 +8,17 @@
 import com.pdsl.specifications.DefaultPhrase;
 import com.pdsl.specifications.FilteredPhrase;
 import com.pdsl.specifications.Phrase;
-import com.pdsl.specifications.PolymorphicDslTransformationException;
 import com.pdsl.testcases.SharedTestCase;
 import com.pdsl.testcases.SharedTestSuite.SharedTestCaseWithInterpreter;
 import com.pdsl.testcases.TestCase;
 import com.pdsl.testcases.TestSection;
-
-import java.util.stream.Collectors;
-
 import org.antlr.v4.runtime.tree.ParseTree;
 import org.antlr.v4.runtime.tree.ParseTreeListener;
 import org.antlr.v4.runtime.tree.ParseTreeVisitor;
 import org.antlr.v4.runtime.tree.ParseTreeWalker;
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
 
-import java.io.IOException;
-import java.io.InputStream;
-import java.nio.charset.Charset;
 import java.util.*;
+import java.util.concurrent.CopyOnWriteArrayList;
 
 /**
  * An executor that runs PDSL tests create from a TestCaseFactory.
@@ -40,8 +31,9 @@
 public class DefaultPolymorphicDslTestExecutor implements TraceableTestRunExecutor, ActivePhraseObservable {
 
     private final ParseTreeWalker walker = new ParseTreeWalker();
-    private static final Charset DEFAULT_CHARSET = Charset.defaultCharset();
-    private final List<ExecutorObserver> activePhraseObservers = new ArrayList<>();
+    // In concurrent test cases an observer might be added to the collection while it's being read from
+    // Use a type that is safe for concurrency.
+    private final List<ExecutorObserver> activePhraseObservers = new CopyOnWriteArrayList<>();
 
     /**
      * Constructs a DefaultPolymorphicDslTestExecutor with a default ColorizedObserver.
diff --git a/src/main/java/com/pdsl/executors/ExecutorObserver.java b/src/main/java/com/pdsl/executors/ExecutorObserver.java
index 67c2f6a..838dabd 100644
--- a/src/main/java/com/pdsl/executors/ExecutorObserver.java
+++ b/src/main/java/com/pdsl/executors/ExecutorObserver.java
@@ -1,7 +1,6 @@
 package com.pdsl.executors;
 
 import com.pdsl.reports.MetadataTestRunResults;
-import com.pdsl.specifications.Phrase;
 import com.pdsl.testcases.SharedTestCase;
 import com.pdsl.testcases.TestCase;
 import com.pdsl.testcases.TestSection;
@@ -52,8 +51,8 @@ default void onAfterPhrase(ParseTreeListener listener,
       ParseTreeWalker walker, TestSection testSection) {}
   
   /** Notifies the implementer of a phrase that has completed execution with a visitor WITHOUT an exception. */
-  void onAfterPhrase(ParseTreeVisitor<?> visitor,
-                     TestSection testSection);
+  default void onAfterPhrase(ParseTreeVisitor<?> visitor,
+                     TestSection testSection) {}
 
    /** Notifies the implementer of a phrase that has failed during excecution with a listener. */
   default void onPhraseFailure(ParseTreeListener listener,