[rules-dev] Declared types - extension

What's the rules around declared types extending another (declared) type? I've been asked to provide for declared type extension in Guvnor (by Prakash) but the Expert user-guide doesn't say it exists at the engine level. I tried "declare MyType extends java.util.ArrayList" and received a parser\compiler error stating "java.util.ArrayList was not a declared type" (or words to that effect) so some form of extension appears possible. Thanks, Mike