code: public @NotNull FileAssert isRelative() {
      isNotNull();
      if (!actual.isAbsolute()) {
        return this;
      }
      failIfCustomMessageIsSet();
      throw failure(format("file:<%s> should be a relative path", actual));
    }
docs: Verifies that the actual {@code File} is a relative path.
  
  @return this assertion object.
  @throws AssertionError if the actual {@code File} is not a relative path.