-+ operator does not work where removed belief is same as added belief

[remcollier]

The following program fails silently on the -+test... line

agent Test {
    types test {
        formula test(string);
    }

    initial test("happy");

    rule +!main(list args) {
        -+test("happy");
    }
}

[remcollier]

Added a check so that, in the case where a belief being removed matches the belief being added, the belief is not removed.