Extending library different behavior and problems

[eluser]minttux[/eluser]
hi
i need to generate log files with special file name so i extended Log.php and create MYO_Log.php inside library in application folder and it worked correctly but it's very clean if i put every extended class or files inside core folder in application
but problem is here
the problem is MYO_log.php has different behavior when i put it in each core or libraries folder

when i put it in libraries it works instead of default Log.php (inside system folder) but when i put in core it doesn't work until i call log functions

is this a bug ?