Trying to compose spock specs with mixins

PrincipalSpec.class

import grails.plugin.spock.UnitSpec;
import org.codehaus.groovy.runtime.ArrayUtil;
import org.codehaus.groovy.runtime.callsite.CallSite;
import org.spockframework.runtime.model.FeatureMetadata;
import org.spockframework.runtime.model.FieldMetadata;
import org.spockframework.runtime.model.SpecMetadata;

@SpecMetadata(filename="PrincipalSpec.groovy")
public class PrincipalSpec extends UnitSpec
{

  @FieldMetadata(name="principalField", ordinal=0, line=14)
  private Object principalField;

  public PrincipalSpec()
  {
    CallSite[] arrayOfCallSite = $getCallSiteArray();
  }

  public void My principal test()
  {
    CallSite[] arrayOfCallSite = $getCallSiteArray();
    arrayOfCallSite[0].call($get$$class$org$spockframework$runtime$SpockRuntime());
    return;
  }

  @FeatureMetadata(name="My principal test", ordinal=0, parameterNames={}, blocks={@org.spockframework.runtime.model.BlockMetadata(kind=org.spockframework.runtime.model.BlockKind.SETUP, texts={}), @org.spockframework.runtime.model.BlockMetadata(kind=org.spockframework.runtime.model.BlockKind.EXPECT, texts={})})
  public void $spock_feature_1_0()
  {
    CallSite[] arrayOfCallSite = $getCallSiteArray(); Object $spock_valueRecorder = arrayOfCallSite[1].callConstructor($get$$class$org$spockframework$runtime$ValueRecorder()); arrayOfCallSite[2].callCurrent(this, "I am the only one");
    arrayOfCallSite[3].callCurrent(this, arrayOfCallSite[4].callCurrent(this));
    arrayOfCallSite[5].call($get$$class$org$spockframework$runtime$SpockRuntime(), ArrayUtil.createArray(arrayOfCallSite[6].call($spock_valueRecorder), "true", Integer.valueOf(18), Integer.valueOf(11), null, arrayOfCallSite[7].call($spock_valueRecorder, $const$1, Boolean.TRUE))); arrayOfCallSite[8].call(arrayOfCallSite[9].callGroovyObjectGetProperty(this)); return;
  }

  public Object getPrincipalField()
  {
    return this.principalField;
  }

  public void setPrincipalField(Object paramObject)
  {
    this.principalField = paramObject;
  }
}

PrincipalSpec.groovy

import grails.plugin.spock.UnitSpec;

@Mixin(ToMixWithPrincipalSpec)
class PrincipalSpec extends UnitSpec {
  def principalField="principalField"
  def "My principal test"() {
		given: println "I am the only one"
		and: println secondaryMethod()
		expect: true
	}
}

/*
grails testApp unit: PrincipalSpec
....
Tests passed: 1
Tests failed: 0

--Output from My principal test--
I am the only one
Hi from helper method of ToMix:principalField

I guess the spock ast trans dont see the feature method of mixed spec (see reverse PrincipalSpec.class)
or i am missing something basic thing 
Too much magic at same time ;-)
It seems a spock extension would be neccesary to make it possible/user friendly
*/

ToMixWithPrincipal.groovy

import grails.plugin.spock.UnitSpec;

class ToMixWithPrincipalSpec extends UnitSpec{
  def "My mixed test" () {
		given: println "hey, i am too"
		expect: true
	}
	
	def secondaryMethod() {
		"Hi from helper method of ToMix:"+principalField
	}
}