Index: src/OFArray.h
==================================================================
--- src/OFArray.h
+++ src/OFArray.h
@@ -84,10 +84,12 @@
 
 /*!
  * @class OFArray OFArray.h ObjFW/OFArray.h
  *
  * @brief An abstract class for storing objects in an array.
+ *
+ * @note Subclasses must implement @ref count and @ref objectAtIndex:.
  */
 @interface OFArray OF_GENERIC(ObjectType): OFObject <OFCopying,
     OFMutableCopying, OFCollection, OFSerialization, OFJSONRepresentation,
     OFMessagePackRepresentation>
 #if !defined(OF_HAVE_GENERICS) && !defined(DOXYGEN)

Index: src/OFCharacterSet.h
==================================================================
--- src/OFCharacterSet.h
+++ src/OFCharacterSet.h
@@ -22,10 +22,12 @@
 
 /*!
  * @class OFCharacterSet OFCharacterSet.h ObjFW/OFCharacterSet.h
  *
  * @brief A class cluster representing a character set.
+ *
+ * @note Subclasses must implement @ref characterIsMember:.
  */
 @interface OFCharacterSet: OFObject
 #ifdef OF_HAVE_CLASS_PROPERTIES
 @property (class, readonly, nonatomic) OFCharacterSet *whitespaceCharacterSet;
 #endif

Index: src/OFCountedSet.h
==================================================================
--- src/OFCountedSet.h
+++ src/OFCountedSet.h
@@ -37,10 +37,13 @@
 /*!
  * @class OFCountedSet OFCountedSet.h ObjFW/OFCountedSet.h
  *
  * @brief An abstract class for a mutable unordered set of objects, counting how
  *	  often it contains an object.
+ *
+ * @note Subclasses must implement @ref countForObject: as well as all methods
+ *	 of @ref OFSet and @ref OFMutableSet that need to be implemented.
  */
 @interface OFCountedSet OF_GENERIC(ObjectType):
     OFMutableSet OF_GENERIC(ObjectType)
 #if !defined(OF_HAVE_GENERICS) && !defined(DOXYGEN)
 # define ObjectType id

Index: src/OFDictionary.h
==================================================================
--- src/OFDictionary.h
+++ src/OFDictionary.h
@@ -49,10 +49,13 @@
  *
  * Keys are copied and thus must conform to the OFCopying protocol.
  *
  * @note Fast enumeration on a dictionary enumerates through the keys of the
  *	 dictionary.
+ *
+ * @note Subclasses must implement @ref objectForKey:, @ref count and
+ *	 @ref keyEnumerator.
  */
 @interface OFDictionary OF_GENERIC(KeyType, ObjectType): OFObject <OFCopying,
     OFMutableCopying, OFCollection, OFSerialization, OFJSONRepresentation,
     OFMessagePackRepresentation>
 #if !defined(OF_HAVE_GENERICS) && !defined(DOXYGEN)

Index: src/OFMutableArray.h
==================================================================
--- src/OFMutableArray.h
+++ src/OFMutableArray.h
@@ -35,10 +35,14 @@
 /*!
  * @class OFMutableArray OFArray.h ObjFW/OFArray.h
  *
  * @brief An abstract class for storing, adding and removing objects in an
  *	  array.
+ *
+ * @note Subclasses must implement @ref insertObject:atIndex:,
+ *	 @ref replaceObjectAtIndex:withObject:, @ref removeObjectAtIndex: as
+ *	 well as all methods of @ref OFArray that need to be implemented.
  */
 @interface OFMutableArray OF_GENERIC(ObjectType): OFArray OF_GENERIC(ObjectType)
 #if !defined(OF_HAVE_GENERICS) && !defined(DOXYGEN)
 # define ObjectType id
 #endif

Index: src/OFMutableDictionary.h
==================================================================
--- src/OFMutableDictionary.h
+++ src/OFMutableDictionary.h
@@ -34,10 +34,14 @@
 
 /*!
  * @class OFMutableDictionary OFDictionary.h ObjFW/OFDictionary.h
  *
  * @brief An abstract class for storing and changing objects in a dictionary.
+ *
+ * @note Subclasses must implement @ref setObject:forKey:,
+ *	 @ref removeObjectForKey: as well as all methods of @ref OFDictionary
+ *	 that need to be implemented.
  */
 @interface OFMutableDictionary OF_GENERIC(KeyType, ObjectType):
     OFDictionary OF_GENERIC(KeyType, ObjectType)
 #if !defined(OF_HAVE_GENERICS) && !defined(DOXYGEN)
 # define KeyType id

Index: src/OFMutableSet.h
==================================================================
--- src/OFMutableSet.h
+++ src/OFMutableSet.h
@@ -21,10 +21,13 @@
 
 /*!
  * @class OFMutableSet OFSet.h ObjFW/OFSet.h
  *
  * @brief An abstract class for a mutable unordered set of unique objects.
+ *
+ * @note Subclasses must implement @ref addObject:, @ref removeObject: as well
+ *	 as all methods of @ref OFSet that need to be implemented.
  */
 @interface OFMutableSet OF_GENERIC(ObjectType): OFSet OF_GENERIC(ObjectType)
 #if !defined(OF_HAVE_GENERICS) && !defined(DOXYGEN)
 # define ObjectType id
 #endif

Index: src/OFSet.h
==================================================================
--- src/OFSet.h
+++ src/OFSet.h
@@ -58,10 +58,13 @@
  *
  * @brief An abstract class for an unordered set of unique objects.
  *
  * @warning Do not mutate objects that are in a set! Changing the hash of
  *	    objects in a set breaks the internal representation of the set!
+ *
+ * @note Subclasses must implement @ref count, @ref containsObject: and
+ *	 @ref objectEnumerator.
  */
 @interface OFSet OF_GENERIC(ObjectType): OFObject <OFCollection, OFCopying,
     OFMutableCopying, OFSerialization>
 #if !defined(OF_HAVE_GENERICS) && !defined(DOXYGEN)
 # define ObjectType id