2048 game and question about FPS in games with CAPI

Hello - 
As an exercise I wrote a version of 2048 game for Lispworks.
 You can find it here
http://www.obrezan.com/lisp/2048/ 
While coding gui with CAPI I run into some difficulties such as how to keep a constant FPS and how to create a game loop with CAPI. Does anyone have some experiences on how to keep FPS on a constant rate while using only CAPI? Also, any examples or thought on how to implement a proper main game loop in CAPI? 
Best, 
 Art 

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