code: func Walk(x Syntax, before, after func(Syntax)) {
  	seen := map[Syntax]bool{
  		nil:           true,
  		(*Decl)(nil):  true,
  		(*Init)(nil):  true,
  		(*Type)(nil):  true,
  		(*Expr)(nil):  true,
  		(*Stmt)(nil):  true,
  		(*Label)(nil): true,
  	}
  	walk(x, before, after, seen)
  }
docs: // Walk traverses the syntax x, calling before and after on entry to and exit from
  // each Syntax encountered during the traversal. In case of cross-linked input,
  // the traversal never visits a given Syntax more than once.