system/classes/Validate.php
public function def ($checkName, $class, $method, $errorMessage, $passing = true, $arguments = false)
    {
        $this->customChecks->$checkName = ['_call', $class, $method, $passing, $arguments];
        $this->lang->$checkName = $errorMessage;
    }
public function def ($checkName, $class, $method, $errorMessage, $passing = true, $arguments = false)
    {
        $this->customChecks->$checkName = ['_call', $class, $method, $passing, $arguments];
        $this->lang->$checkName = $errorMessage;
    }
Stores a customly defined validation check.