(eval 28), as far as i know, means the 28th eval that has been processed. As for what value, well, you will just have to inspect that yourself. I recommend printing the the value to see what it is, just remove the eval and run it. Then, you should have a clue as to which variable is undefined.

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)