Fix Set examples for 4.0 (#69)

* 4.0: Fix typo in class name for Set example

* 4.0: Inherit from correct Set classes in Set examples

The first change is necessary for the example to work at all. The lower
two changes are not strictly necessary though, but they may help when
trying or copying parts of the example.

Author: meineerde

4.0.md

@@ -8,9 +8,9 @@ description: Ruby 4.0 full and annotated changelog
 # Ruby 4.0
 
 * **Released at:** Dec 25, 2025 (<a class="github" href="https://github.com/ruby/ruby/blob/v4.0.0/NEWS.md">NEWS.md</a> file)
-* **Status (as of Dec 26, 2025):** 4.0 is just released
+* **Status (as of Jan 09, 2026):** 4.0 is just released
 * **This document first published:** Dec 26, 2025
-* **Last change to this document:** Dec 26, 2025
+* **Last change to this document:** Jan 09, 2026
 
 <!--
 * **Reason:**
@@ -339,7 +339,7 @@ By redefinining `#instance_variables_to_inspect`, the default output of `#inspec
 
   # Now, as methods of SubclassCompatible are implemented mostly as they were before,
   # old assumptions work:
-  class SmallStringSet
+  class SmallStringSet < Set
     def initialize(items) = super(items.to_a[...5])
     def add(item) = super(item.to_s)
   end
@@ -352,7 +352,7 @@ By redefinining `#instance_variables_to_inspect`, the default output of `#inspec
   # Note that if your code relied on `@hash` instance variable directly
   # (which it shouldn't have), it will be broken despite the compatibility
   # layer:
-  class SmallStringsSet
+  class SmallStringSet < Set
     # Somebody decided using internal variable would be more efficient
     def numeric? = @hash.keys.all?(Numeric)
   end
@@ -371,7 +371,7 @@ By redefinining `#instance_variables_to_inspect`, the default output of `#inspec
 
   # It would be faster, but the assumptions which methods calls which
   # internally aren't necessary correct anymore
-  class SmallStringSet2
+  class SmallStringSet2 < Set::CoreSet
     def initialize(items) = super(items.to_a[...5])
     def add(item) = super(item.to_s)
   end

_src/4.0.md

@@ -339,7 +339,7 @@ By redefinining `#instance_variables_to_inspect`, the default output of `#inspec
 
   # Now, as methods of SubclassCompatible are implemented mostly as they were before,
   # old assumptions work:
-  class SmallStringSet
+  class SmallStringSet < Set
     def initialize(items) = super(items.to_a[...5])
     def add(item) = super(item.to_s)
   end
@@ -352,7 +352,7 @@ By redefinining `#instance_variables_to_inspect`, the default output of `#inspec
   # Note that if your code relied on `@hash` instance variable directly
   # (which it shouldn't have), it will be broken despite the compatibility
   # layer:
-  class SmallStringsSet
+  class SmallStringSet < Set
     # Somebody decided using internal variable would be more efficient
     def numeric? = @hash.keys.all?(Numeric)
   end
@@ -371,7 +371,7 @@ By redefinining `#instance_variables_to_inspect`, the default output of `#inspec
 
   # It would be faster, but the assumptions which methods calls which
   # internally aren't necessary correct anymore
-  class SmallStringSet2
+  class SmallStringSet2 < Set::CoreSet
     def initialize(items) = super(items.to_a[...5])
     def add(item) = super(item.to_s)
   end