/**
   * A graph consisting of a single root {@code a}:
   *
   * {@snippet :
   * a
   * }
   */
  private static final SuccessorsFunction<Character> SINGLE_ROOT = createSingleRootGraph();

  /**
   * A graph that is not a tree (for example, it has two antiparallel edge between {@code e} and
   * {@code f} and thus has a cycle) but is a valid input to {@link Traverser#forTree} when starting