horn :fp.datalog.generate_explanations :fp.xform.karr Assertion violation at ../src/muz/rel/dl_mk_explanations.cpp Line: 865

Hi,
For this formula:
```
(set-option :fp.datalog.generate_explanations true)
(set-option :fp.xform.karr true)
(check-sat-using horn)
```
Z3  throws out an assertion violation:
```
ASSERTION VIOLATION
File: ../src/muz/rel/dl_mk_explanations.cpp
Line: 865
union_fun
(C)ontinue, (A)bort, (S)top, (T)hrow exception, Invoke (G)DB
a
```

OS: Ubuntu 18.04
Commit: 9e7af79
