i have run the simulation step-by-step for this test case and the expected answer is correct. the final state is (with head at H, going leftwards and hence colliding):

-----------$--$----$-
--$-------$-$$------$
$----$$-$--$-$-$--$--
$---$--$-----$-------
-$$$---$----$$$--$---
$--$$---$----$$-$-$--
-$--$$$--oHoo$-$-----
$---$----oooo--------
-----$-------$-$$--$$
-$$-$---$-$-$$$--$-$-
-$$----$-$$----$-$$-$
$--$-$---$-$-----$-$$
-$$$---$-----$$$----$

I think your code must get lost in some infinite loop cause I don't think you may exceed the maximum recursion limit with a reasonnable code here. Hard to help you without more elements

"Brilliant?" Are you UK-based too?

Anyway, thanks :)