code: public String getProjectedNodeKey( String externalNodeKey ) {
          Projection projection = snapshot.get().getProjectionForExternalNode(externalNodeKey);
          return projection != null ? projection.getProjectedNodeKey() : null;
      }
docs: Returns the key of the internal (federated) node which has been projected on the external node with the given key.
  
  @param externalNodeKey a {@code non-null} String representing the {@link NodeKey} format an external node
  @return either a {@code non-null} String representing the node key of the projected node, or {@code null} if there is no
  projection.