Data update

Task/Constrained-genericity/Pluto/constrained-genericity.pluto

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+-- Abstract class.
+class eatable
+    function eat() end -- override in child class
+end
+
+class foodbox extends eatable
+    private contents
+
+    function __construct(contents)
+        if !contents:checkall(|c| -> c instanceof eatable) then
+            error("All foodbox elements must be eatable.")
+        end
+        self.contents = contents
+    end
+
+    function getContents() return self.contents end
+end
+
+-- Inherits from eatable and overrides eat() method.
+class pie extends eatable
+    function __construct(private filling) end
+
+    function eat()
+        print($"{self.filling} pie, yum!")
+    end
+end
+
+-- Not an eatable
+class bicycle end
+
+local items = {new pie("Apple"), new pie("Gooseberry")}
+local fb = new foodbox(items)
+fb:getContents():foreach(|item| -> item:eat())
+print()
+items:insert(new bicycle())
+fb = new foodbox(items)  -- throws an error because bicycle not eatable