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diff --git a/org.argeo.api/src/org/argeo/api/gcr/ContentNamespace.java b/org.argeo.api/src/org/argeo/api/gcr/ContentNamespace.java
new file mode 100644
index 000000000..a59557180
--- /dev/null
+++ b/org.argeo.api/src/org/argeo/api/gcr/ContentNamespace.java
@@ -0,0 +1,106 @@
+package org.argeo.api.gcr;
+
+import java.net.URI;
+import java.net.URISyntaxException;
+import java.nio.charset.StandardCharsets;
+import java.util.Objects;
+import java.util.UUID;
+
+/**
+ * An XML-style namespace with a related UUID v3.
+ * 
+ * @see https://www.w3.org/TR/xml-names/
+ */
+public class ContentNamespace {
+	private final UUID uuid;
+	private final URI uri;
+
+	public ContentNamespace(String uri) {
+		try {
+			this.uri = new URI(uri);
+		} catch (URISyntaxException e) {
+			throw new IllegalArgumentException("Cannot interpret " + uri + " as URI", e);
+		}
+		this.uuid = namespaceUuid(this.uri);
+	}
+
+	public ContentNamespace(URI uri) {
+		this.uri = uri;
+		this.uuid = namespaceUuid(uri);
+	}
+
+	ContentNamespace(URI uri, UUID uuid) {
+		this.uri = uri;
+		assert uuid.equals(namespaceUuid(uri));
+		this.uuid = uuid;
+	}
+
+	/** Empty namespace */
+	private ContentNamespace() {
+		try {
+			this.uri = new URI("");
+		} catch (URISyntaxException e) {
+			throw new IllegalStateException("Cannot create empty URI");
+		}
+		this.uuid = NIL_UUID;
+	}
+
+	public UUID getUuid() {
+		return uuid;
+	}
+
+	public URI getUri() {
+		return uri;
+	}
+
+	public UUID nameUuid(String name) {
+		return nameUuid(getUuid(), name);
+	}
+
+
+	public final static UUID NIL_UUID = UUID.fromString("00000000-0000-0000-0000-000000000000");
+	public final static ContentNamespace EMPTY_NS = new ContentNamespace();
+
+	/**
+	 * The UUID v3 of http://www.w3.org/2000/xmlns/ within the standard DNS
+	 * namespace, to be used as a base for the namespaces.
+	 * 
+	 * @see https://www.w3.org/TR/xml-names/#ns-decl
+	 */
+	// uuidgen --md5 --namespace @dns --name http://www.w3.org/2000/xmlns/
+	// NOTE : must be declared before default namespaces
+	public final static UUID XMLNS_UUID = UUID.fromString("4b352aad-ba1c-3139-b9d3-41e5816f6088");
+	public final static ContentNamespace CR_NS = new ContentNamespace("http://argeo.org/ns/cr");
+
+
+	public static UUID namespaceUuid(URI namespaceUri) {
+		Objects.requireNonNull(namespaceUri, "Namespace URI cannot be null");
+		return nameUUIDv3(XMLNS_UUID, namespaceUri.toString().getBytes(StandardCharsets.UTF_8));
+	}
+
+	public static UUID nameUuid(UUID namespace, String name) {
+		Objects.requireNonNull(namespace, "Namespace cannot be null");
+		Objects.requireNonNull(namespace, "Name cannot be null");
+		return nameUUIDv3(namespace, name.getBytes(StandardCharsets.UTF_8));
+	}
+
+	/*
+	 * CANONICAL IMPLEMENTATION based on java.util.UUID.nameUUIDFromBytes(byte[])
+	 */
+	private static UUID nameUUIDv3(UUID namespace, byte[] name) {
+		byte[] arr = new byte[name.length + 16];
+		copyUuidBytes(namespace, arr, 0);
+		System.arraycopy(name, 0, arr, 16, name.length);
+		return UUID.nameUUIDFromBytes(arr);
+	}
+
+	private static void copyUuidBytes(UUID uuid, byte[] arr, int offset) {
+		long msb = uuid.getMostSignificantBits();
+		long lsb = uuid.getLeastSignificantBits();
+		assert arr.length >= 16 + offset;
+		for (int i = offset; i < 8 + offset; i++)
+			arr[i] = (byte) ((msb >> ((7 - i) * 8)) & 0xff);
+		for (int i = 8 + offset; i < 16 + offset; i++)
+			arr[i] = (byte) ((lsb >> ((15 - i) * 8)) & 0xff);
+	}
+}