@Test
public void split_canSplit() {

  assertThat(first('=').split(" foo=bar", (String k, String v) -> k)).isEqualTo(" foo");
  assertThat(first('=').split("foo=bar ", (String k, String v) -> v)).isEqualTo("bar ");
  assertThat(first('=').split(" foo=bar", (String k, String v) -> k)).isEqualTo(" foo");
}