How to share common assertions between tests in Spock Framework?

Simple test illustrating helper methods to share assertions between tests.

package com.farenda.solved

import spock.lang.Specification

class ListWithHelperTest extends Specification {

    def "should be empty after removing element"() {
        given:
        def list = [42] // it's a List

        when:
        list.remove(0)

        then:
        // Spock can assert than an Exception was _NOT_ thrown:
        notThrown(IllegalArgumentException)
        // can call helper method here:
        doCommonAssertions(list)
    }

    // Assertion helpers have to have void return type and explicit asserts.
    def void doCommonAssertions(List<Integer> list) { // type is optional
        assert list != null
        assert list.isEmpty()
    }
}