code: func (nodes *orderedNodes) addAt(i int, node *node) *node {
  	if i == len(*nodes) {
  		*nodes = append(*nodes, node)
  		return nil
  	}
  
  	if (*nodes)[i].value == node.value {
  		overwritten := (*nodes)[i]
  		// this is a duplicate, there can't be a duplicate
  		// point in the last dimension
  		(*nodes)[i] = node
  		return overwritten
  	}
  
  	*nodes = append(*nodes, nil)
  	copy((*nodes)[i+1:], (*nodes)[i:])
  	(*nodes)[i] = node
  	return nil
  }
docs: // addAt will add the provided node at the provided index.  Returns
  // a node if one was overwritten.