@CanIgnoreReturnValue
  public PathSubject isAbsolute() {
    if (!actual.isAbsolute()) {
      failWithActual(simpleFact("expected to be absolute"));
    }
    return this;
  }
Asserts that the path is absolute (it has a root component).
jimfs/src/test/java/com/google/common/jimfs/PathSubject.java