diff --git a/compiler/java/com/google/dart/compiler/CommandLineOptions.java b/compiler/java/com/google/dart/compiler/CommandLineOptions.java
index b5ea083eabc..ea18b66652c 100644
--- a/compiler/java/com/google/dart/compiler/CommandLineOptions.java
+++ b/compiler/java/com/google/dart/compiler/CommandLineOptions.java
@@ -4,9 +4,9 @@
 
 package com.google.dart.compiler;
 
+import com.google.common.base.Joiner;
 import com.google.common.collect.Lists;
 import com.google.dart.runner.DartRunner;
-import com.google.dart.runner.RunnerOptions;
 
 import org.kohsuke.args4j.Argument;
 import org.kohsuke.args4j.CmdLineException;
@@ -42,19 +42,18 @@ public class CommandLineOptions {
             usage = "Debugging: disable type optimizations")
     private boolean disableTypeOptimizations = false;
 
-    @Option(name = "--documentation-lib", aliases = { "-documentation-lib" },
+    @Option(name = "-documentation-lib",
         usage = "only generate documentation for the given library")
     private String documentationLibrary = null;
 
-    @Option(name = "--documentation-out", aliases = { "-documentation-out" },
-        usage = "directory to receive documentation output")
+    @Option(name = "-documentation-out", usage = "directory to receive documentation output")
     private String documentationOutputDirectory = null;
 
-    @Option(name = "--generate-documentation", aliases = { "-generate-documentation" },
+    @Option(name = "-generate-documentation",
         usage = "generate documentation for the provided source files")
     private boolean generateDocumentation = false;
 
-    @Option(name = "--generate-isolate-stubs", aliases = { "-generate-isolate-stubs" },
+    @Option(name = "-generate-isolate-stubs",
         usage = "classes to generate stubs for, comma-separated")
     private String generateIsolateStubs = null;
 
@@ -65,63 +64,50 @@ public class CommandLineOptions {
     @Option(name = "--ignore-unrecognized-flags", usage = "ignore unrecognized command line flags")
     private boolean ignoreUnrecognizedFlags = false;
 
-    @Option(name = "--isolate-stub-out", aliases = { "-isolate-stub-out" },
-        usage = "file to receive generated stub output")
+    @Option(name = "-isolate-stub-out", usage = "file to receive generated stub output")
     private String isolateStubOutputFile = null;
 
-    @Option(name = "--jvm-metrics-detail", usage = "summary or verbose (default is summary)")
+    @Option(name = "-jvm-metrics-detail", usage = "summary or verbose (default is summary)")
     private String jvmMetricDetail = "summary";
 
-    @Option(name = "--jvm-metrics-format", usage = "tabular or pretty (default is tabular)")
+    @Option(name = "-jvm-metrics-format", usage = "tabular or pretty (default is tabular)")
     private String jvmMetricFormat = "tabular";
 
-    @Option(name = "--jvm-metrics-type", usage = "comma-separated list, including:\n"
+    @Option(name = "-jvm-metrics-type", usage = "comma-separated list, including:\n"
         + "  all:  show all available stat types (default)\n"
         + "  gc:   show garbage collection stats\n"
         + "  mem:  show memory stats\n" + "  jit:  show jit stats")
     private String jvmMetricType = "all";
 
-    @Option(name = "--noincremental", aliases = { "-noincremental" },
-        usage = "disable incremental compilation")
+    @Option(name = "-noincremental", usage = "disable incremental compilation")
     private boolean noincremental = false;
 
+    // see shouldOptimize() below
     private boolean optimize = false;
 
-    /**
-     * Enables optimization of the generated JavaScript.
-     */
-    @Option(name = "--optimize", aliases = { "-optimize" }, usage = "produce optimized code")
-    public void setOptimize(boolean optimize) {
-      this.optimize = optimize;
-    }
-
-    @Option(name = "--out", usage = "write generated JavaScript to the specified file")
-    private File outputFilename = null;
-
     // TODO(zundel): -out is for backward compatibility until scripts are updated
     @Option(name = "--work", aliases = { "-out" }, usage = "directory to receive compiler output")
     private File workDirectory = new File("out");
 
-    @Option(name = "--help", aliases = { "-?", "-help" }, usage = "prints this help message")
+    @Option(name = "-help", usage = "prints this help message")
     private boolean showHelp = false;
 
-    @Option(name = "--jvm-metrics", usage = "print jvm metrics at end of compilation")
+    @Option(name = "-jvm-metrics", usage = "print jvm metrics at end of compilation")
     private boolean showJvmMetrics = false;
 
-    @Option(name = "--metrics", usage = "print compilation metrics")
+    @Option(name = "-metrics", usage = "print compilation metrics")
     private boolean showMetrics = false;
 
+    @Argument
+    private final List<String> sourceFiles = new ArrayList<String>();
+
     @Option(name = "--fatal-type-errors", aliases = { "-fatal-type-errors" },
         usage = "type errors are fatal errors (instead of warnings)")
     private boolean typeErrorsAreFatal = false;
 
-    @Option(name = "--fatal-warnings", aliases = { "-Werror" },
-        usage = "warnings (excluding type warnings) are fatal errors")
+    @Option(name = "-Werror", usage = "warnings (excluding type warnings) are fatal errors")
     private boolean warningsAreFatal = false;
 
-    @Argument
-    private final List<String> sourceFiles = new ArrayList<String>();
-
     /**
      * Returns whether the option -check-only is provided.
      */
@@ -194,11 +180,11 @@ public class CommandLineOptions {
     /**
      * Returns whether the compiler should attempt to incrementally recompile.
      */
-    public boolean buildIncrementally() {
+    public boolean incremental() {
       return !noincremental;
     }
 
-    public boolean shouldBatch() {
+    public boolean isBatch() {
       return batch;
     }
 
@@ -210,6 +196,17 @@ public class CommandLineOptions {
       return generateHumanReadableOutput;
     }
 
+    /**
+     * Enables optimization of the generated JavaScript.
+     */
+    @Option(name = "-optimize", aliases = { "--optimize" }, usage = "produce optimized code")
+    public void optimize(boolean optimize) {
+      this.optimize = optimize;
+    }
+
+    @Option(name = "--out", usage = "write generated JavaScript to the specified file")
+    private File outputFilename = null;
+
     /**
      * @return the path to receive compiler output.
      */
@@ -257,7 +254,7 @@ public class CommandLineOptions {
   /**
    * Command line options accepted by the {@link DartRunner} entry point.
    */
-  public static class DartRunnerOptions extends CompilerOptions implements RunnerOptions {
+  public static class DartRunnerOptions extends CompilerOptions {
 
     @Option(name = "--compile-only", usage = "compile but do not execute")
     private boolean compileOnly = false;
@@ -265,19 +262,18 @@ public class CommandLineOptions {
     @Option(name = "--expose_core_impl", usage = "automatic import of dart:coreimpl library")
     private boolean exposeCoreImpl = false;
 
-    @Option(name = "--verbose", usage = "extra diagnostic output")
-    private boolean verbose = false;
-
     @Option(name="--prof", usage = "enable profiling")
     private boolean prof;
 
     @Option(name = "--rhino", usage = "use rhino as the JavaScript interpreter")
     private boolean rhino = false;
 
+    @Option(name = "--verbose", usage = "extra diagnostic output")
+    private boolean verbose = false;
+
     /**
      * @return <code>true</code> if the program should compile but not execute.
      */
-    @Override
     public boolean shouldCompileOnly() {
       return compileOnly;
     }
@@ -292,7 +288,6 @@ public class CommandLineOptions {
     /**
      * Returns <code>true</code> if profiling is enabled.
      */
-    @Override
     public boolean shouldProfile() {
       return prof;
     }
@@ -300,7 +295,6 @@ public class CommandLineOptions {
     /**
      * @return <code>true</code> if rhino should be used as the runtime (default is to invoke d8)
      */
-    @Override
     public boolean useRhino() {
       return rhino;
     }
@@ -308,7 +302,6 @@ public class CommandLineOptions {
     /**
      * @return <code>true</code> to enable diagnostic output
      */
-    @Override
     public boolean verbose() {
       return verbose;
     }
@@ -318,7 +311,7 @@ public class CommandLineOptions {
   /**
    * Command line options accepted by the {@link TestRunner} entry point.
    */
-  public static class TestRunnerOptions extends DartRunnerOptions  {
+  public static class TestRunnerOptions extends DartRunnerOptions {
   }
 
   /**
@@ -350,6 +343,7 @@ public class CommandLineOptions {
     }
     CmdLineParser cmdLineParser = new CmdLineParser(parsedOptions);
     for (int i = 0, len = args.length; i < len; i++) {
+      System.out.println("Parsing: " + Joiner.on(" ").join(args));
       try {
         cmdLineParser.parseArgument(args);
       } catch (CmdLineException e) {
@@ -362,7 +356,8 @@ public class CommandLineOptions {
           List<String> newArgs = Lists.newArrayList();
           for (String arg : args) {
             if (arg.equals(option)) {
-              System.out.println("(Ignoring unrecognized flag: " + arg + ")");
+              // TODO(zundel): remove diagnostic output
+              System.out.println("Ignoring unrecognized flag: " + arg);
               continue;
             }
             newArgs.add(arg);
diff --git a/compiler/java/com/google/dart/compiler/DartCompiler.java b/compiler/java/com/google/dart/compiler/DartCompiler.java
index 37b52de956a..2334e3f3e96 100644
--- a/compiler/java/com/google/dart/compiler/DartCompiler.java
+++ b/compiler/java/com/google/dart/compiler/DartCompiler.java
@@ -783,7 +783,7 @@ public class DartCompiler {
     CompilerOptions topCompilerOptions = processCommandLineOptions(args);
     boolean result = false;
     try {
-      if (topCompilerOptions.shouldBatch()) {
+      if (topCompilerOptions.isBatch()) {
         if (args.length > 1) {
           System.err.println("(Extra arguments specified with -batch ignored.)");
         }
@@ -791,7 +791,7 @@ public class DartCompiler {
           @Override
           public boolean invoke(String[] args) throws Throwable {
             CompilerOptions compilerOptions = processCommandLineOptions(args);
-            if (compilerOptions.shouldBatch()) {
+            if (compilerOptions.isBatch()) {
               System.err.println("-batch ignored: Already in batch mode.");
             }
             return compilerMain(compilerOptions);
diff --git a/compiler/java/com/google/dart/compiler/DefaultCompilerConfiguration.java b/compiler/java/com/google/dart/compiler/DefaultCompilerConfiguration.java
index 24023b454b8..ecb305ad97a 100644
--- a/compiler/java/com/google/dart/compiler/DefaultCompilerConfiguration.java
+++ b/compiler/java/com/google/dart/compiler/DefaultCompilerConfiguration.java
@@ -162,7 +162,7 @@ public class DefaultCompilerConfiguration implements CompilerConfiguration {
 
   @Override
   public boolean incremental() {
-    return compilerOptions.buildIncrementally();
+    return compilerOptions.incremental();
   }
 
   @Override
diff --git a/compiler/java/com/google/dart/runner/DartRunner.java b/compiler/java/com/google/dart/runner/DartRunner.java
index b63e1c2efae..613ca9a9c96 100644
--- a/compiler/java/com/google/dart/runner/DartRunner.java
+++ b/compiler/java/com/google/dart/runner/DartRunner.java
@@ -9,6 +9,7 @@ import com.google.common.io.CharStreams;
 import com.google.common.io.Files;
 import com.google.dart.compiler.Backend;
 import com.google.dart.compiler.CommandLineOptions;
+import com.google.dart.compiler.CommandLineOptions.CompilerOptions;
 import com.google.dart.compiler.CommandLineOptions.DartRunnerOptions;
 import com.google.dart.compiler.CompilerConfiguration;
 import com.google.dart.compiler.DartArtifactProvider;
@@ -45,8 +46,10 @@ import java.net.URI;
 import java.nio.charset.Charset;
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.EnumSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 
 public class DartRunner {
@@ -99,6 +102,26 @@ public class DartRunner {
 
     LibrarySource app = new UrlLibrarySource(new File(script));
 
+    // TODO(zundel): Replace RunnerFlag enum with DartRunnerOptions
+    final Set<RunnerFlag> flags = EnumSet.noneOf(RunnerFlag.class);
+    if (options.shouldOptimize()) {
+      flags.add(RunnerFlag.OPTIMIZE);
+    }
+    if (options.verbose()) {
+      flags.add(RunnerFlag.VERBOSE);
+    }
+    if (options.shouldProfile()) {
+      flags.add(RunnerFlag.PROFILE);
+    }
+    if (options.shouldCompileOnly()) {
+      flags.add(RunnerFlag.COMPILE_ONLY);
+    }
+    if (options.typeErrorsAreFatal()) {
+      flags.add(RunnerFlag.FATAL_TYPE_ERRORS);
+    }
+    if (options.useRhino()) {
+      flags.add(RunnerFlag.USE_RHINO);
+    }
     if (options.shouldExposeCoreImpl()) {
       imports = new ArrayList<LibrarySource>(imports);
       // use a place-holder LibrarySource instance, to be replaced when embedded
@@ -119,7 +142,7 @@ public class DartRunner {
       };
 
     CompilationResult compiled;
-    compiled = compileApp(app, imports, options, listener);
+    compiled = compileApp(app, imports, flags, listener);
 
     if (listener.getProblemCount() != 0) {
       throw new RunnerError("Compilation failed.");
@@ -150,9 +173,8 @@ public class DartRunner {
       }
     }
 
-    if (!options.shouldCompileOnly()) {
-      runApp(compiled, app.getName(), options, scriptArguments.toArray(new String[0]),
-             stdout, stderr);
+    if (!flags.contains(RunnerFlag.COMPILE_ONLY)) {
+      runApp(compiled, app.getName(), flags, scriptArguments.toArray(new String[0]), stdout, stderr);
     }
   }
 
@@ -269,7 +291,7 @@ public class DartRunner {
   }
 
   public static void compileAndRunApp(LibrarySource app,
-                                      DartRunnerOptions options,
+                                      Set<RunnerFlag> flags,
                                       CompilerConfiguration config,
                                       DartCompilerListener listener,
                                       String[] dartArguments,
@@ -278,21 +300,20 @@ public class DartRunner {
       throws RunnerError {
     CompilationResult compiled = compileApp(
         app, Collections.<LibrarySource>emptyList(), config, listener);
-    runApp(compiled, app.getName(), options, dartArguments, stdout, stderr);
+    runApp(compiled, app.getName(), flags, dartArguments, stdout, stderr);
   }
 
   private static void runApp(CompilationResult compiled,
                              String sourceName,
-                             DartRunnerOptions options,
+                             Set<RunnerFlag> flags,
                              String[] scriptArguments,
                              PrintStream stdout,
                              PrintStream stderr)
       throws RunnerError {
-
-    if (options.useRhino()) {
-      new RhinoLauncher().execute(compiled.js, sourceName, scriptArguments, options, stdout, stderr);
+    if (flags.contains(RunnerFlag.USE_RHINO)) {
+      new RhinoLauncher().execute(compiled.js, sourceName, scriptArguments, flags, stdout, stderr);
     } else {
-      new V8Launcher(compiled.mapping).execute(compiled.js, sourceName, scriptArguments, options,
+      new V8Launcher(compiled.mapping).execute(compiled.js, sourceName, scriptArguments, flags,
                                                stdout, stderr);
     }
   }
@@ -307,15 +328,22 @@ public class DartRunner {
     }
   }
 
-  private static CompilationResult compileApp (LibrarySource app, List<LibrarySource> imports,
-      final DartRunnerOptions options, DartCompilerListener listener) throws RunnerError {
+  private static CompilationResult compileApp(LibrarySource app,
+                                              List<LibrarySource> imports,
+                                              final Set<RunnerFlag> flags,
+                                              DartCompilerListener listener)
+      throws RunnerError {
+    // TODO(johnlenz): create a "OptimizingCompilerConfiguration"
+
     Backend backend;
-    if (options.shouldOptimize()) {
+    CompilerOptions defaultOptions = new CompilerOptions();
+    if (flags.contains(RunnerFlag.OPTIMIZE)) {
       backend = new ClosureJsBackend();
+      defaultOptions.optimize(true);
     } else {
       backend = new JavascriptBackend();
     }
-    CompilerConfiguration config = new DefaultCompilerConfiguration(backend, options) {
+    CompilerConfiguration config = new DefaultCompilerConfiguration(backend, defaultOptions) {
       @Override
       public boolean expectEntryPoint() {
         return true;
@@ -323,7 +351,7 @@ public class DartRunner {
 
       @Override
       public boolean typeErrorsAreFatal() {
-        return options.typeErrorsAreFatal();
+        return flags.contains(RunnerFlag.FATAL_TYPE_ERRORS);
       }
     };
     return compileApp(app, imports, config, listener);
diff --git a/compiler/java/com/google/dart/runner/JavaScriptLauncher.java b/compiler/java/com/google/dart/runner/JavaScriptLauncher.java
index 550315984aa..d1821177120 100644
--- a/compiler/java/com/google/dart/runner/JavaScriptLauncher.java
+++ b/compiler/java/com/google/dart/runner/JavaScriptLauncher.java
@@ -5,6 +5,7 @@
 package com.google.dart.runner;
 
 import java.io.PrintStream;
+import java.util.Set;
 
 /**
  * @author floitsch@google.com (Florian Loitsch)
@@ -12,7 +13,7 @@ import java.io.PrintStream;
  */
 interface JavaScriptLauncher {
 
-  void execute(String jsScript, String sourceName, String[] args, RunnerOptions flags,
+  void execute(String jsScript, String sourceName, String[] args, Set<RunnerFlag> flags,
                PrintStream stdout, PrintStream stderr)
       throws RunnerError;
 
diff --git a/compiler/java/com/google/dart/runner/RhinoLauncher.java b/compiler/java/com/google/dart/runner/RhinoLauncher.java
index 21f12b692b3..1c927607646 100644
--- a/compiler/java/com/google/dart/runner/RhinoLauncher.java
+++ b/compiler/java/com/google/dart/runner/RhinoLauncher.java
@@ -13,6 +13,7 @@ import org.mozilla.javascript.Undefined;
 
 import java.io.PrintStream;
 import java.lang.reflect.Member;
+import java.util.Set;
 
 /**
  * @author floitsch@google.com (Florian Loitsch)
@@ -177,7 +178,7 @@ public class RhinoLauncher implements JavaScriptLauncher {
   }
 
   @Override
-  public void execute(String jsScript, String sourceName, String[] args, RunnerOptions options,
+  public void execute(String jsScript, String sourceName, String[] args, Set<RunnerFlag> flags,
                       PrintStream stdout, PrintStream stderr)
       throws RunnerError {
     try {
@@ -201,7 +202,7 @@ public class RhinoLauncher implements JavaScriptLauncher {
     } catch (RhinoException e) {
       // TODO(jgw): This is a hack to dump the translated source when something goes wrong. It can
       // be removed as soon as we have a source map we can use to provide source-level errors.
-      if (options.verbose()) {
+      if (flags.contains(RunnerFlag.VERBOSE)) {
         stdout.println(jsScript);
         stdout.flush();
       }
diff --git a/compiler/java/com/google/dart/runner/RunnerFlag.java b/compiler/java/com/google/dart/runner/RunnerFlag.java
new file mode 100644
index 00000000000..84bc9198886
--- /dev/null
+++ b/compiler/java/com/google/dart/runner/RunnerFlag.java
@@ -0,0 +1,17 @@
+// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+package com.google.dart.runner;
+
+/**
+ * Flags that can be given to Runners and Launchers.
+ */
+public enum RunnerFlag {
+  VERBOSE,
+  PROFILE,
+  OPTIMIZE,
+  COMPILE_ONLY,
+  USE_RHINO,
+  FATAL_TYPE_ERRORS;
+}
diff --git a/compiler/java/com/google/dart/runner/TestRunner.java b/compiler/java/com/google/dart/runner/TestRunner.java
index 3f9901289fc..adfb1d455d9 100644
--- a/compiler/java/com/google/dart/runner/TestRunner.java
+++ b/compiler/java/com/google/dart/runner/TestRunner.java
@@ -32,7 +32,7 @@ public class TestRunner {
     try {
       boolean runBatch = false;
       TestRunnerOptions options = processCommandLineOptions(args);
-      if (options.shouldBatch()) {
+      if (options.isBatch()) {
         runBatch = true;
         if (args.length > 1) {
           System.err.println("(Extra arguments specified with -batch ignored.)");
diff --git a/compiler/java/com/google/dart/runner/V8Launcher.java b/compiler/java/com/google/dart/runner/V8Launcher.java
index de55d20f56f..ed52706adef 100644
--- a/compiler/java/com/google/dart/runner/V8Launcher.java
+++ b/compiler/java/com/google/dart/runner/V8Launcher.java
@@ -20,6 +20,7 @@ import java.io.StringWriter;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
+import java.util.Set;
 
 /**
  * @author floitsch@google.com (Florian Loitsch)
@@ -69,7 +70,7 @@ public class V8Launcher implements JavaScriptLauncher {
   }
 
   @Override
-  public void execute(String jsScript, String sourceName, String[] args, RunnerOptions options,
+  public void execute(String jsScript, String sourceName, String[] args, Set<RunnerFlag> flags,
                       PrintStream stdout, PrintStream stderr)
       throws RunnerError {
     if (!isConfigured()) {
@@ -85,7 +86,7 @@ public class V8Launcher implements JavaScriptLauncher {
       ArrayList<String> command = new ArrayList<String>();
       command.add(v8Executable().getAbsolutePath());
       command.add(sourceFile.getAbsolutePath());
-      if (options.shouldProfile()) {
+      if (flags.contains(RunnerFlag.PROFILE)) {
         command.add("--prof");
       }
       command.add("--");
@@ -132,7 +133,7 @@ public class V8Launcher implements JavaScriptLauncher {
       if (exitValue != 0) {
         StringWriter stringWriter = new StringWriter();
         PrintWriter out = new PrintWriter(stringWriter);
-        if (options.verbose()) {
+        if (flags.contains(RunnerFlag.VERBOSE)) {
           out.println(jsScript);
         }
         out.println("Execution failed.");
diff --git a/compiler/javatests/com/google/dart/compiler/end2end/End2EndTestCase.java b/compiler/javatests/com/google/dart/compiler/end2end/End2EndTestCase.java
index 2f41f9d6fe1..682e19acdbf 100644
--- a/compiler/javatests/com/google/dart/compiler/end2end/End2EndTestCase.java
+++ b/compiler/javatests/com/google/dart/compiler/end2end/End2EndTestCase.java
@@ -4,8 +4,6 @@
 
 package com.google.dart.compiler.end2end;
 
-import com.google.dart.compiler.CommandLineOptions;
-import com.google.dart.compiler.CommandLineOptions.DartRunnerOptions;
 import com.google.dart.compiler.CompilerConfiguration;
 import com.google.dart.compiler.CompilerTestCase;
 import com.google.dart.compiler.DartCompilerListener;
@@ -19,9 +17,11 @@ import com.google.dart.compiler.backend.js.ClosureJsBackend;
 import com.google.dart.compiler.backend.js.JavascriptBackend;
 import com.google.dart.runner.DartRunner;
 import com.google.dart.runner.RunnerError;
+import com.google.dart.runner.RunnerFlag;
 
 import org.mozilla.javascript.RhinoException;
 
+import java.util.EnumSet;
 import java.util.List;
 
 /**
@@ -71,21 +71,14 @@ public abstract class End2EndTestCase extends CompilerTestCase {
   protected void runTest(LibrarySource app, OptimizationLevel opLevel,
                          DartCompilerListener listener) {
     try {
-      final CompilerConfiguration config = getCompilerConfiguration(opLevel);
-      DartRunnerOptions verboseOptions = new CommandLineOptions.DartRunnerOptions() {
-        @Override
-        public boolean verbose() {
-          return true;
-        }
-      };
-
+      CompilerConfiguration config = getCompilerConfiguration(opLevel);
       DartRunner.compileAndRunApp(app,
-                                  verboseOptions,
-                                  config,
-                                  listener,
-                                  new String[0],
-                                  System.out,
-                                  System.err);
+                                EnumSet.of(RunnerFlag.VERBOSE),
+                                config,
+                                listener,
+                                new String[0],
+                                System.out,
+                                System.err);
     } catch (RhinoException e) {
       // TODO(jgw): This is a hack to dump the translated source when something goes wrong. It can
       // be removed as soon as we have a source map we can use to provide source-level errors.