Error "rule graph is not stratifiable" in Grakn - reasoning

Question

I'm trying to insert this rule (data is masked):
when {
$d isa person;
$a isa animal;
$r (role-1: $d, role-2: $a) isa relation-1, has attr $ds;
not {$r1 (role-3: $d, role-4: $a) isa relation-2; };
$ds > 0.5;
}, then {
(role-3: $a, role-4: $d, role-5: $ds) isa relation-2;
};
However I keep getting this structural validation error after I do commit. Please help
INVALID_ARGUMENT: InvalidKBException-A structural validation error has occurred. Please correct the [`1`] errors found.
The rule graph is not stratifiable - it contains following cycles with negation: [[[Base Type [RELATION_TYPE] - Id [V65648] - Label [relation-2] ]]]
. Please check server logs for the stack trace.
All uncommitted data is cleared

Answer 1

Rules that cause loops - for example, where the conclusion is negated in the when clause - are not allowed in Grakn.
Here's an example to demonstrate why.
define
person sub entity, plays employee;
employment sub relation, relates employee;
there-are-no-unemployed sub rule,
when {
$p isa person;
not {
(employee: $p) isa employment;
};
}, then {
(employee: $p) isa employment;
}
If we have a person who is not in an employment relation, then they are in an employment relation. But now, we've contradicted the initial premise of the rule - that they were not in an employment relation. Thus, we have a logical contradiction.