option(debug_compile,on) preceding any rules in the file
containing the handler to enable CHR debugging. The CHR debugging
mechanism works by instrumenting the code generated by the CHR
compiler. Basically, the CHR debugger works like the Prolog
debugger. The main differences are: there are extra ports
specific to CHR, and the CHR debugger provides no means for the user to
change the flow of control, i.e. there are currently no retry
and fail options available.