+/**
+ * Mark elements as "seen" using a hash of the element. Not supported
+ * by all block plugins.
+ *
+ * @param bg group to update
+ * @param seen_results results already seen
+ * @param seen_results_count number of entries in @a seen_results
+ */
+typedef void
+(*GNUNET_BLOCK_GroupMarkSeenFunction)(struct GNUNET_BLOCK_Group *bg,
+ const struct GNUNET_HashCode *seen_results,
+ unsigned int seen_results_count);
+
+
+/**
+ * Merge two groups, if possible. Not supported by all block plugins,
+ * can also fail if the nonces were different.
+ *
+ * @param bg1 group to update
+ * @param bg2 group to merge into @a bg1
+ * @return #GNUNET_OK on success, #GNUNET_NO if the nonces were different and thus
+ * we failed.
+ */
+typedef int
+(*GNUNET_BLOCK_GroupMergeFunction)(struct GNUNET_BLOCK_Group *bg1,
+ const struct GNUNET_BLOCK_Group *bg2);
+
+
+/**
+ * Serialize state of a block group.
+ *
+ * @param bg group to serialize
+ * @param[out] nonce set to the nonce of the @a bg
+ * @param[out] raw_data set to the serialized state
+ * @param[out] raw_data_size set to the number of bytes in @a raw_data
+ * @return #GNUNET_OK on success, #GNUNET_NO if serialization is not
+ * supported, #GNUNET_SYSERR on error
+ */
+typedef int
+(*GNUNET_BLOCK_GroupSerializeFunction)(struct GNUNET_BLOCK_Group *bg,
+ uint32_t *nonce,
+ void **raw_data,
+ size_t *raw_data_size);
+
+
+/**
+ * Destroy resources used by a block group.
+ *
+ * @param bg group to destroy, NULL is allowed
+ */
+typedef void
+(*GNUNET_BLOCK_GroupDestroyFunction)(struct GNUNET_BLOCK_Group *bg);
+
+
+/**
+ * Block group data. The plugin must initialize the callbacks
+ * and can use the @e internal_cls as it likes.
+ */
+struct GNUNET_BLOCK_Group
+{
+
+ /**
+ * Context owning the block group. Set by the main block library.
+ */
+ struct GNUENT_BLOCK_Context *ctx;
+
+ /**
+ * Type for the block group. Set by the main block library.
+ */
+ enum GNUNET_BLOCK_Type type;
+
+ /**
+ * Serialize the block group data, can be NULL if
+ * not supported.
+ */
+ GNUNET_BLOCK_GroupSerializeFunction serialize_cb;
+
+ /**
+ * Function to call to mark elements as seen in the group.
+ * Can be NULL if not supported.
+ */
+ GNUNET_BLOCK_GroupMarkSeenFunction mark_seen_cb;
+
+ /**
+ * Function to call to merge two groups.
+ * Can be NULL if not supported.
+ */
+ GNUNET_BLOCK_GroupMergeFunction merge_cb;
+
+ /**
+ * Function to call to destroy the block group.
+ * Must not be NULL.
+ */
+ GNUNET_BLOCK_GroupDestroyFunction destroy_cb;
+
+ /**
+ * Internal data structure of the plugin.
+ */
+ void *internal_cls;
+
+};
+
+
+/**
+ * Create a new block group.
+ *
+ * @param ctx block context in which the block group is created
+ * @param type type of the block for which we are creating the group
+ * @param nonce random value used to seed the group creation
+ * @param raw_data optional serialized prior state of the group, NULL if unavailable/fresh
+ * @param raw_data_size number of bytes in @a raw_data, 0 if unavailable/fresh
+ * @param va variable arguments specific to @a type
+ * @return block group handle, NULL if block groups are not supported
+ * by this @a type of block (this is not an error)
+ */
+typedef struct GNUNET_BLOCK_Group *
+(*GNUNET_BLOCK_GroupCreateFunction)(void *cls,
+ enum GNUNET_BLOCK_Type type,
+ uint32_t nonce,
+ const void *raw_data,
+ size_t raw_data_size,
+ va_list va);
+
+