public void TestEquals()
        {
            UnaryExpressionStub expression1 = new UnaryExpressionStub(new VariableExpression(new Variable("scooby")));
            UnaryExpressionStub expression2 = new UnaryExpressionStub(new VariableExpression(new Variable("scooby")));
            UnaryExpressionStub expression3 = new UnaryExpressionStub(new VariableExpression(new Variable("velma")));

            Assert.IsTrue(expression1.Equals(expression2), "expression1 should equal expression2");
            Assert.IsTrue(!expression1.Equals(expression3), "expression1 should not equal expression3");
        }