title: No resource identifier found for attribute 'layout_constraintLeft_toLeftOf' in package
body: <p>I am using the Android SDK 2.2.3. I am following a book on programming Android.</p>
  
  <p>The code is:</p>
  
  <pre><code>&lt;android.support.constraint.ConstraintLayout
      xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:app="http://schemas.android.com/apk/res-auto"
      xmlns:tools="http://schemas.android.com/tools"
      android:id="@+id/activity_main"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      tools:context="eu.a123sandberg.webview.MainActivity"
      tools:layout_editor_absoluteX="0dp"
      tools:layout_editor_absoluteY="81dp"&gt;
  
      &lt;WebView
          android:layout_width="384dp"
          android:layout_height="511dp"
          tools:layout_editor_absoluteX="0dp"
          tools:layout_editor_absoluteY="0dp"
          app:layout_constraintLeft_toLeftOf="@+id/activity_main"
          tools:layout_constraintLeft_creator="0"
          app:layout_constraintTop_toTopOf="@+id/activity_main"
          tools:layout_constraintTop_creator="0"
          app:layout_constraintRight_toRightOf="@+id/activity_main"
          tools:layout_constraintRight_creator="0"
          app:layout_constraintBottom_toBottomOf="@+id/activity_main"
          tools:layout_constraintBottom_creator="0"
          /&gt;
  
  &lt;/android.support.constraint.ConstraintLayout&gt;
  </code></pre>
  
  <p>I have tried three different examples with the android.support.constraint.ConstraintLayout and in all cases do I get the same error message:</p>
  
  <pre><code>Error:(13) No resource identifier found for attribute 'layout_constraintLeft_toLeftOf' in package 'eu.a123sandberg.webview'
  </code></pre>
  
  <p>The build.gradle file contains:</p>
  
  <pre><code>apply plugin: 'com.android.application'
  
  android {
      compileSdkVersion 25
      buildToolsVersion "25.0.2"
      defaultConfig {
          applicationId "eu.a123sandberg.webview"
          minSdkVersion 21
          targetSdkVersion 25
          versionCode 1
          versionName "1.0"
                                                                                    testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
      }
      buildTypes {
          release {
              minifyEnabled false
              proguardFiles getDefaultProguardFile('proguard-                   android.txt'), 'proguard-rules.pro'
          }
       }
  }
  
  dependencies {
      compile fileTree(dir: 'libs', include: ['*.jar'])
      androidTestCompile('com.android.support.test.espresso:espresso-            core:2.2.2', {
      exclude group: 'com.android.support', module: 'support-annotations'
      })
      compile 'com.android.support:appcompat-v7:25.1.1'
      testCompile 'junit:junit:4.12'
  }
  </code></pre>
  
  <p>What do I have to do to resolve the error?</p>
  
  <p>I have googled the error and not been able to find a suggestion that resolves the error.</p>
  
  <p>Please help. </p>