Failure in invIncompleteGamma = gamma^-1:

NaN looks suspiciously

```
*** Failed! Falsifiable (after 42 tests and 3 shrinks):
-1.0e-3
-39.015782564394144
a    = 1.0e-3
p    = 1.5782564394143606e-2
x    = 0.0
p'   = 0.0
err  = 1.0
pred = NaN
(used seed -46209551813637022)
```