code: private boolean nodeOk( GridNode node ) {
          return node != null && !assignedFlowsMap.isMarked(node.col, node.row);
      }
docs: Checks if the node is ok.
  
  <p>A node is ok if:</p>
  <ul>
  <li>if the node is valid (!= null in surrounding)</li>
  <li>if the node has not been processed already (!.isMarked)</li>
  </ul>