Negated Atom with Constant in Body causes unexpected behavior

[matzemathics]

Following program

a(1) :- ~b(1).

is normalized to

a(1) :- ~b(?x), ?x = 1.

Now ?x is a unsafe variable, which the variable order generation code is not prepared to handle, leading to a runtime error:

Variable order must contain an entry for every variable.

This is one half of the problem in #428.