The "guile" scheme interpreter has a stepping option. Take care to pick a new release of guile, because rational numbers have only been incorporated into the main release since Feb 2006. From experience, minlog will need some minor tweaking (where accidentally not RSR5-compliant). 

You can run the "guile" interpreter interactively or just by "input | guile | python parse_debugger", to get pie charts "input | guile | python parse_debugger_pie.py"