code: public static String getChildUnder(String path, String parentPath) {
          int start = parentPath.length();
          String local = path.substring(start, path.length());
          String name = getFirstPathComponent(local);
          return name;
      }
docs: Answer the first path element of a path which follows a leading sub-path.
  
  For example, for path "/grandParent/parent/child/grandChild" and
  leading sub-path "/grandParent/parent", answer "child".
  
  The result is unpredictable if the leading path does not start the
  target path, and does not reach a separator character in the target
  path. An exception will be thrown if the leading path is longer
  than the target path.
  
  @param path The path from which to obtain a path element.
  @param leadingPath A leading sub-path of the target path.
  
  @return The first path element of the target path following the
  leading sub-path.