Merging keys with yaml

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>

	<groupId>com.example</groupId>
	<artifactId>yaml-merge-keys</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<packaging>jar</packaging>

	<name>yaml-merge-keys</name>
	<description>Demo project for Spring Boot</description>

	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>2.0.0.RELEASE</version>
		<relativePath/> <!-- lookup parent from repository -->
	</parent>

	<properties>
		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
		<project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
		<java.version>1.8</java.version>
	</properties>

	<dependencies>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter</artifactId>
		</dependency>

		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-test</artifactId>
			<scope>test</scope>
		</dependency>
	</dependencies>

	<build>
		<plugins>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
			</plugin>
		</plugins>
	</build>


</project>

src_main_java_com_example_ymlmerge_ProptestApplication.java

package com.example.ymlmerge;

import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class ProptestApplication implements CommandLineRunner{

	public static void main(String[] args) {
		SpringApplication.run(ProptestApplication.class, args);
	}

	@Override
	public void run(String... args) throws Exception {

	}
}

src_main_resources_application.yml

a:
  b: foo

<<a: # merge with previous declaration of key 'a'
  c: bar

src_test_java_com_example_ymlmerge_ProptestApplicationTests.java

package com.example.ymlmerge;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

import static junit.framework.TestCase.assertEquals;
import static junit.framework.TestCase.assertNotNull;

@RunWith(SpringRunner.class)
@SpringBootTest
public class ProptestApplicationTests {

	@Value("${a.b}")
	private String b;

	@Value("${a.c}")
	private String c;

	@Test
	public void contextLoads() {
		assertEquals("foo", b);
		assertEquals("bar", c);
	}

}