modifyStack f = modifyState (`modifyStack'` f)
                where rec `modifyStack'` f = rec { iStack = f (iStack rec) }
modifyStack f = modifyState (`modifyStack'` f)
                where rec `modifyStack'` f = rec { iStack = f (iStack rec) }
modifyStack f = modifyState (`modifyStack'` f)
                where rec `modifyStack'` f = rec { iStack = f (iStack rec) }