public Negation setOperand(Predicate operand) {
        Objects.requireNonNull(operand, "operand");
        this.operand = operand;
        return this;
    }
@Test
    void requireThatEqualsIsImplemented() {
        Negation lhs = new Negation(SimplePredicates.newString("foo"));
        assertEquals(lhs, lhs);
        assertNotEquals(lhs, new Object());

        Negation rhs = new Negation(SimplePredicates.newString("bar"));
        assertNotEquals(lhs, rhs);
        rhs.setOperand(SimplePredicates.newString("foo"));
        assertEquals(lhs, rhs);
    }