code: @Override
    protected boolean hasUnderflow(FlatRStarTreeNode node) {
      if(node.isLeaf()) {
        return node.getNumEntries() < leafMinimum;
      }
      else {
        return false;
      }
    }
docs: Returns true if in the specified node an underflow occurred, false
  otherwise.
  
  @param node the node to be tested for underflow
  @return true if in the specified node an underflow occurred, false
  otherwise