negation error with constants inside the negated atom

[Laschoking]

Using constants in negated atoms does not work, when there is more than 1 negated atom per rule.
In the example I use string constants, but it is the same with integers.

1. Error, when using one constant "a" in 2 negated atoms:

C(?a,?a0,?a1):-
    A(?a,?a0,?a1),
    ~ B(?a,?a0),
    ~ B("a",?a1).

2. Error, when using two constants in 2 negated atoms:

C(?a,?a0,?a1):-
    A(?a,?a0,?a1),
    ~B("a",?a0),
    ~B(?"a",?a1).

The error message is:

thread 'main' panicked at nemo/src/execution/planning/arithmetic.rs:38:26:
Variable order must contain an entry for every variable.

The error also triggers, when the two negated atoms are referring to different relations (i.e. replace the second negated atom with ~D("a",?a1) and it will still not work).

[matzemathics]

this is likely also related to #446, which definitely needs to be addressed for the next release