code: private void merge(YamlNode source, YamlNode target) {
          if (source == null) {
              return;
          }
  
          checkAmbiguousConfiguration(source, target);
  
          if (isMapping(source)) {
              mergeMappingNodes(asMapping(source), asMapping(target));
          } else if (isSequence(source)) {
              mergeSequenceNodes(asSequence(source), asSequence(target));
          }
      }
docs: Merges the source YAML document into the target YAML document
  <p/>
  If a given source node is not found in the target, it will be attached
  If a given source node is found in the target, this method is invoked
  recursively with the given node
  
  @param source The source YAML document's root
  @param target The target YAML document's root