= core
  * db
    * http://codeigniter.com/user_guide/database/active_record.html
    * se pueden concatenar: ''$query = $this->db->order_by(campo)->get(tabla)''
    * ''$this->db->select('<campo>,<campo>,<campo>');''
    * ''$this->db->select_max('<campo>','<alias>');''
    * ''$this->db->select_min('<campo>','<alias>');''
    * ''$this->db->select_avg('<campo>','<alias>');''
    * ''$this->db->select_sum('<campo>','<alias>');''
    * ''$this->db->distinct()''
    * ''$this->db->from('<tabla>');''
    * ''$this->db->join('<tabla>','<condicion_join>');''
    * ''$this->db->count_all('<tabla>')''
    * ''$this->db->order_by('<campo>','asc|desc')''
    * ''$this->db->group_by('<campo>','asc|desc')''
    * ''$this->db->where('<campo>','valor')''
    * ''$this->db->where_in('<campo>','array_valores')''
    * ''$this->db->where_not_in('<campo>','array_valores')''
    * ''$this->db->or_where_in('<campo>','array_valores')''
    * ''$this->db->or_where_not_in('<campo>','array_valores')''
    * ''$this->db->or_where('<campo>','valor')''
    * ''$this->db->like('<campo>','valor')''
    * ''$query = $this->db->get('<tabla>',limite,offset)''
      * ''$query->result()''
      * ''$query->num_rows''
    * ''$query = $this->db->getwhere('<tabla>',array('<campo>'=><variable>,'<campo>'=>variable);'' <- auna el where y el get en una instrucción
    * ''$this->db->insert('<tabla>',<datos>);''
      * ''$this->db_insert_id()''
    * ''$this->db->update('<tabla>',<datos>);'' <- utilizar antes ''$this->db->where()''
    * ''$this->db->delete('<tabla>');'' <- utilizar antes ''$this->db->where()''
  * output
    * ''$this->output->enable_profiler(TRUE);''
  * benchmark
    * ''$this->benchmark->mark('<marca>');''
    * ''$this->benchmark->elapsed_time('<marca_inicio>','<marca_final>');''
  * router
    * ''$this->router->method'' -> último método llamado