kw override

ImplMember  = { Attr } [ Vis ] ( Receiver ( "fn" | "op" ) Sig ( Block | "=>" Expr )
            | "override" "fn" Sig ( Block | "=>" Expr ) | "open" "fn" Sig ( Block | "=>" Expr )
            | "init" Sig ( Block | "=>" Expr ) | "dep" Ident ":" Type | Rpc | Const ) ;

data Animal { name: str }
impl Animal {
  open fn speak() -> str => "..."
  fn hello() -> str => "I am {name}: {speak()}"
}

data Dog { embed Animal; breed: str }
impl Dog { override fn speak() -> str => "Woof, {super.speak()}" }

Dog{ name: "Rex", breed: "lab" }.hello()    // "I am Rex: Woof, ..."