[Haskell-cafe] Debugging with ghci

For a long time I've worked under the assumption that a breakpoint can only be used once in a ghci session, because if you run the function (main say) again, it runs to completion, ignoring the breakpoint. Restarting ghci and resetting the break point resolves the problem, but does not explain why a breakpoint cannot be used more than once. How can this behavior be explained? Also, when I stop at a breakpoint :step works as expected, but :trace does not? It simply runs the rest of the function, and using :back subsequently results in the message "not stopped at a breakpoint." Any comments on this behavior? Thanks, Dominick