Incr Tcl

   package require Itcl
   
   itcl::class Toaster {
       variable crumbs 0
       method toast {nslices} {
           if {$crumbs > 50} {
               error "== FIRE! FIRE! =="
           }
           set crumbs [expr $crumbs+4*$nslices]
       }
       method clean {} {
           set crumbs 0
       }
   }
   
   itcl::class SmartToaster {
       inherit Toaster
       method toast {nslices} {
           if {$crumbs > 40} {
               clean
           }
           return [chain $nslices]
       }
   }
   
   set toaster [SmartToaster #auto]
   $toaster toast 2