public void addOperator(Transformation<?> transformation) {
        checkNotNull(transformation, "transformation must not be null.");
        this.transformations.add(transformation);
    }
@Test
    void testAddOperator() {
        ExecutionEnvironmentImpl env =
                new ExecutionEnvironmentImpl(
                        new DefaultExecutorServiceLoader(), new Configuration(), null);

        TestingTransformation<Integer> t1 = new TestingTransformation<>("t1", Types.INT, 10);
        TestingTransformation<String> t2 = new TestingTransformation<>("t2", Types.STRING, 5);
        env.addOperator(t1);
        env.addOperator(t2);
        assertThat(env.getTransformations()).containsExactly(t1, t2);
    }