code: public function find($criteria)
  	{
  		if (!$criteria)
  		{
  			error_log("collection error: no criteria specified");
  			return null;
  		}
  
  		else if (is_callable($criteria))
  		{
  			foreach ($this->models as $model)
  				if ($criteria($model)) return $model;
  		}
  		else if (is_a($criteria, "SnooPHP\Model\Model"))
  		{
  			foreach ($this->models as $i => $model)
  				if ($model == $criteria) return $i;
  		}
  
  		return null;
  	}
docs: Find an element
  
  @param Callable|Model $criteria model to match or callable called on every element (return true if element match the criteria)
  
  @return Model|int|null	if model is given, return index or null if not found.
  If callable given, return matching element or null if none found.
  In any case returns only a single result