public function setMapping( $severity, $pipe )
    {
        if ( !isset( $this->mapping[$severity] ) )
        {
            throw new \RuntimeException( "Unknown severity: " . $severity );
        }

        if ( ( $pipe !== self::SILENCE ) &&
             ( $pipe !== self::STDOUT ) &&
             ( $pipe !== self::STDERR ) )
        {
            throw new \RuntimeException( "Unknown output pipe: " . $pipe );
        }

        $this->mapping[$severity] = $pipe;
    }