Predicate invention does not work when directions are not given.

[andrewcropper]

There is a problem with predicate invention. When directions are given for background relations, predicate invention does not work.

[yangdinglou]

Here is my observation on this issue. It seems buggy in both cases, because the order of sub clauses matters. We don't know which variables should be grounded, so the bad order will result in error even the candidate model is exactly what we want. It could be unsolvable in theory when the directions are not given; but for the case when directions are provided, it's possible to calculate the partial direction for the invented predicate and enumerate all cases.