assertThat(sources.precedenceOf(PropertySource.named("a")), is(0));
assertThat(sources.precedenceOf(PropertySource.named("b")), is(1));
assertThat(sources.precedenceOf(PropertySource.named("c")), is(2));
assertThat(sources.precedenceOf(PropertySource.named("d")), is(3));
assertThat(sources.precedenceOf(PropertySource.named("e")), is(4));
assertThat(sources.precedenceOf(PropertySource.named("f")), is(5));
assertThat(sources.precedenceOf(PropertySource.named("g")), is(6));
assertEquals(sources.remove("a"), PropertySource.named("a"));
assertThat(sources.size(), equalTo(6));
assertThat(sources.contains("a"), is(false));