Code using library(clpfd) can be debugged with the usual
debugger, but it does not capture all relevant aspects of constraint
execution: the propagation cascade and domain changes are not visible.
To capture such aspects, a separate, dedicated debugger is available;
see lib-fdbg.
The v command (print variable bindings) of the usual debugger can be handy. It will endeavor to print the variable bindings of the clause containing the current goal, as well as any goals that are blocked on a variable found among those bindings. In particular, it will show the current domains of such variables. See Debug Commands.