You can use several methods...end-methods sections. The only things you can do to the class in these sections are: defining methods, and overriding the class's selectors. You must not define new selectors or fields.
Note that you often have to override a selector before using it. In particular, you usually have to override construct with a new method before you can invoke heap-new and friends. E.g., you must not create a circle before the overrides construct sequence in the example above.