code: public static Matcher<Tree> parentNode(Matcher<? extends Tree> treeMatcher) {
      @SuppressWarnings("unchecked") // Safe contravariant cast
      Matcher<Tree> matcher = (Matcher<Tree>) treeMatcher;
      return new ParentNode(matcher);
    }
docs: Matches an AST node if its parent node is matched by the given matcher. For example, {@code
  parentNode(kindIs(Kind.RETURN))} would match the {@code this} expression in {@code return
  this;}