Added deque.

deque/.project

@@ -0,0 +1,28 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<projectDescription>
+	<name>deque</name>
+	<comment></comment>
+	<projects>
+	</projects>
+	<buildSpec>
+		<buildCommand>
+			<name>org.eclipse.jdt.core.javabuilder</name>
+			<arguments>
+			</arguments>
+		</buildCommand>
+	</buildSpec>
+	<natures>
+		<nature>org.eclipse.jdt.core.javanature</nature>
+	</natures>
+	<filteredResources>
+		<filter>
+			<id>1733232021426</id>
+			<name></name>
+			<type>30</type>
+			<matcher>
+				<id>org.eclipse.core.resources.regexFilterMatcher</id>
+				<arguments>node_modules|\.git|__CREATED_BY_JAVA_LANGUAGE_SERVER__</arguments>
+			</matcher>
+		</filter>
+	</filteredResources>
+</projectDescription>

deque/.settings/org.eclipse.core.resources.prefs

@@ -0,0 +1,2 @@
+eclipse.preferences.version=1
+encoding/<project>=UTF-8

deque/.settings/org.eclipse.jdt.core.prefs

@@ -0,0 +1,11 @@
+eclipse.preferences.version=1
+org.eclipse.jdt.core.compiler.codegen.targetPlatform=21
+org.eclipse.jdt.core.compiler.codegen.unusedLocal=preserve
+org.eclipse.jdt.core.compiler.compliance=21
+org.eclipse.jdt.core.compiler.debug.lineNumber=generate
+org.eclipse.jdt.core.compiler.debug.localVariable=generate
+org.eclipse.jdt.core.compiler.debug.sourceFile=generate
+org.eclipse.jdt.core.compiler.problem.enablePreviewFeatures=disabled
+org.eclipse.jdt.core.compiler.problem.reportPreviewFeatures=warning
+org.eclipse.jdt.core.compiler.release=enabled
+org.eclipse.jdt.core.compiler.source=21

deque/src/deque/Main.java

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+package deque;
+
+import tester.Tester;
+
+class Examples {
+
+    Sentinel<String> exampleSentinel;
+
+    void init() {
+        exampleSentinel = new Sentinel<String>();
+    }
+
+    void testSentinel(Tester t) {
+        init();
+        t.checkExpect(exampleSentinel.nxt, exampleSentinel);
+    }
+}
+
+// A doubly-linked list element.
+abstract class ANode<PointsTo> {
+
+    PointsTo nxt; // Next element.
+    PointsTo pre; // Previous element.
+
+    ANode(PointsTo nxt, PointsTo pre) {
+        this.nxt = nxt;
+        this.pre = pre;
+    }
+}
+
+// Self-referential node.
+abstract class ASRefNode {
+
+    ASRefNode nxt;
+    ASRefNode pre;
+
+    ASRefNode() {
+        this.nxt = this;
+        this.pre = this;
+    }
+}
+
+class Deque<A> {
+
+    Sentinel<A> sentinel;
+
+    Deque(Sentinel<A> sentinel) {
+        this.sentinel = sentinel;
+    }
+}
+
+class Sentinel<T> extends ANode<ANode<T>> {
+
+    Sentinel() {
+        super(null, null);
+        this.nxt = this;
+    }
+}
+
+class Node<T> extends ANode<Node<T>> {
+
+    T val;
+
+    Node(T val) {
+        super(null, null);
+        this.val = val;
+    }
+}