docs: private void insertFringeMigrate(
                                      InsertStack       stack,
                                      GBSNode           p,
                                      Object            mkey)
      {
          GBSNode endp = p;
          int endIndex = stack.index();
          /* Maximum number of right child nodes   */
          /*  before the fringe must be rebalanced */
          int maxBal = maximumFringeImbalance();
  
          stack.setMigratingKey(mkey);
          if (mkey != null)              /* Have a key to migrate                 */
          {
              stack.processSubFringe(p); /* See InsertStack.processNode           */
              endp = stack.lastNode();
              endIndex = stack.lastIndex();
          }
          if (stack.migrating())         /* Need a right child for migrating key  */
          {
              _xno++;
              endp.addRightLeaf(stack.migratingKey());
          }
  
          insertCheckFringeBalance(
                                  stack,
                                  endp,
                                  endIndex,
                                  maxBal);
  
      }