How to eliminate code duplication in "then" block

I am using spock testing framework. say I have a lot of tests with similar structure like this:

def "test"() {
    when:
    doSomething()
    then:
    1 * mock.method1(...)
    2 * mock.method2(...)
}

I want to move the code of "then" block to a helper method:

def assertMockMethodsInvocations() {
    1 * mock.method1(...)
    2 * mock.method2(...)
}

Then I can invoke this helper method to eliminate the code duplications in my test like:

def "test"() {
    when:
    doSomething()
    then:
    assertMockMethodsInvocations()
}

However method invocations cannot be captured when you put n * mock.method(...) in a helper method.