code: Set<PolicyNodeImpl> getPolicyNodesValid(int depth, String validOID) {
          HashSet<PolicyNodeImpl> set = new HashSet<>();
  
          if (mDepth < depth) {
              for (PolicyNodeImpl node : mChildren) {
                  set.addAll(node.getPolicyNodesValid(depth, validOID));
              }
          } else {
              if (mValidPolicy.equals(validOID))
                  set.add(this);
          }
  
          return set;
      }
docs: Finds all nodes at the specified depth that contains the
  specified valid OID
  
  @param depth an int representing the desired depth
  @param validOID a String encoding the valid OID to match
  @return a Set of matched <code>PolicyNode</code>s