function children($idx=-1)
    {
        if ($idx===-1)
        {
            return $this->children;
        }
        if (isset($this->children[$idx]))
        {
            return $this->children[$idx];
        }
        return null;
    }
function children($idx=-1)
    {
        if ($idx===-1)
        {
            return $this->children;
        }
        if (isset($this->children[$idx]))
        {
            return $this->children[$idx];
        }
        return null;
    }
function children ( $ idx = - 1 ) { if ( $ idx === - 1 ) { return $ this -> children ; } if ( isset ( $ this -> children [ $ idx ] ) ) { return $ this -> children [ $ idx ] ; } return null ; }
returns children of node
returns children of node