@Test public void tag_mutableSpan_empty() {

  when(parseValue.apply(input, context)).thenReturn("");
  tag.tag(input, context, mutableSpan);
  verify(parseValue).apply(input, context);
  verifyNoMoreInteractions(parseValue); // doesn't parse twice
  MutableSpan expected = new MutableSpan();
  expected.tag("key", "");
  assertThat(mutableSpan).isEqualTo(expected);
}