#4316 difapply mostly untested in testsuite

[Raymond Toy]

I modified difapply to insert a break after the first cond
clause, just before $noprincipal is set. When running the
testsuite, the break is never reached.

We should add some tests for the other cases in difapply.

[Raymond Toy]

difapply is called from mtorat and ztorat which is called from ztoinf. This functions sets the variable s to (deg-var d ivar) which is eventually passed to difapply. The first test in difapply is:

     (cond ((eq ($asksign (m+ (deg-var d ivar) (m- s) (m- 2.)))  '$neg)
        (return nil)))

Since s is (deg-var d ivar), the arg to $asksign is basically s-s-2 or -2. Thus we always exit difapply here and never execute the rest of the function.

Am I missing something? Is there some other way to get past this first clause?

[Raymond Toy]

Note also that difapply is only called when *dflag* is NIL. This is normally the case because *dflag* is only set to T in diffhk, which is only called by difapply at the very end, which we never reach. dintegrate always binds *dflag* to NIL.