question: @ Override protected void addToFrontier ( Node node ) { if ( ! explored . contains ( node . getState ( ) ) && ! frontierStates . contains ( node . getState ( ) ) ) { frontier . add ( node ) ; frontierStates . add ( node . getState ( ) ) ; updateMetrics ( frontier . size ( ) ) ; } }
target:  Inserts the node at the tail of the frontier if the corresponding state is not already a frontier state and was not yet explored.