code: public void addParentNode(final String childNodeName, final String parentNodeName) {
      checkIsBuilt();
  
      final Node child = this.nameToNodeMap.get(childNodeName);
      if (child == null) {
        throw new DagException(String.format("Unknown child node (%s). Did you create the node?",
            childNodeName));
      }
  
      final Node parent = this.nameToNodeMap.get(parentNodeName);
      if (parent == null) {
        throw new DagException(
            String.format("Unknown parent node (%s). Did you create the node?", parentNodeName));
      }
  
      child.addParent(parent);
    }
docs: Add a parent node to a child node. All the names should have been registered with this builder
  with the {@link DagBuilder#createNode(String, NodeProcessor)} call.
  
  @param childNodeName name of the child node
  @param parentNodeName name of the parent node