doc-src/IsarAdvanced/Codegen/Thy/examples/class.ML

 struct
 
 type 'a null = {null : 'a};
 fun null (A_:'a null) = #null A_;
 
-fun head B_ (x :: xs) = x
-  | head B_ [] = null B_;
+fun head A_ (x :: xs) = x
+  | head A_ [] = null A_;
 
 val null_option : 'a option = NONE;
 
 fun null_optiona () = {null = null_option} : ('a option) null;