static boolean prefixMatches(Tree target, Tree pattern) {
    if (pattern.isEmpty()) return true;
    if (target.isEmpty() || target.value() != pattern.value()) return false;

    return prefixMatches(target.left(), pattern.left())
        && prefixMatches(target.right(), pattern.right());
  }
  /**
   * Search the tree for the first instance of `pattern`, giving preference to the left child.
   *
   * <p>`pattern` is a tree, representing a pattern of nodes. `NilNode`s should be considered
   * wildcards.
   *
   * <p>Example patterns ---------------- `Tree()` Matches any tree.
   *
   * <p>`Tree(3)` Matches any tree where the root node has a value of `3`.
   *
   * <p>`Tree(3, Tree(2), Tree())` Matches any tree where the root node has a value of `3`, and a
   * left sub-tree with a root value of `2`, and any (or no) right sub-tree.
   */
  static Optional<Tree> find(Tree target, Tree pattern) {
    if (pattern.isEmpty()) return Optional.of(target);
    if (target.isEmpty()) return Optional.empty();
    if (prefixMatches(target, pattern)) return Optional.of(target);

    Optional<Tree> leftMatch = find(target.left(), pattern);
    if (leftMatch.isPresent()) return leftMatch;
    return find(target.right(), pattern);
  }