Discrepancies Between Different Modes

[julmb]

While investigating #108, I came across some more discrepancies between differentiation modes.

>>> import Numeric.AD.Mode.Forward as F
>>> import Numeric.AD.Mode.Reverse as R
>>> F.diff cos 0
>>> R.diff cos 0
-0.0
0.0

>>> import Numeric.AD.Mode.Forward as F
>>> import Numeric.AD.Mode.Reverse as R
>>> let f x = log (1 - exp x)
>>> F.diff (F.diff f) 0
>>> R.diff (R.diff f) 0
-Infinity
NaN

These examples may be somewhat contrived, but given the nature of algebra, I feel like any discrepancy could probably be amplified to yield significantly different results given the right circumstances.