assertThat(sources.size(), equalTo(7));
assertThat(sources.precedenceOf(PropertySource.named("a")), is(0));
assertThat(sources.precedenceOf(PropertySource.named("b")), is(1));
assertThat(sources.precedenceOf(PropertySource.named("c")), is(2));
sources.replace("a", new MockPropertySource("a-replaced"));
assertThat(sources.size(), equalTo(7));
assertThat(sources.precedenceOf(PropertySource.named("a-replaced")), is(0));
assertThat(sources.precedenceOf(PropertySource.named("b")), is(1));
assertThat(sources.precedenceOf(PropertySource.named("c")), is(2));