code: public SpringApplicationBuilder parent(Class<?>... sources) {
  		if (this.parent == null) {
  			this.parent = new SpringApplicationBuilder(sources)
  					.web(WebApplicationType.NONE).properties(this.defaultProperties)
  					.environment(this.environment);
  		}
  		else {
  			this.parent.sources(sources);
  		}
  		return this.parent;
  	}
docs: Add a parent application with the provided sources. Default args and environment
  are copied up into the parent, but everything else is a clean sheet.
  @param sources the sources for the application (Spring configuration)
  @return the parent builder