Exercise 2.84: Using the raise operation
of Exercise 2.83, modify the apply-generic procedure so that it
coerces its arguments to have the same type by the method of successive
raising, as discussed in this section.  You will need to devise a way to test
which of two types is higher in the tower.  Do this in a manner that is
“compatible” with the rest of the system and will not lead to problems in
adding new levels to the tower.