Atoum How to mock a function

myClass.php

<?php

class myClass {
  
  protected $creator;
  
  public function set(callable $creator)
  {
    $this->creator = $creator
  }
  
  public function callIt($id)
  {
    return call_user_func($this->creator, $id);
  }
}

test.php

<?php

namespace mageekguy\atoum;

class test extends atoum\test
{
  public function testCall()
  {
    $this
      ->given(
        $this->newTestedInstance,
        $this->function->dummy = function ($a) { return $a.$a;},
        $this->testedInstance->set($this->function->dummy)     // Error "must be callable, object given"
      )
      ->if($this->testedInstance->callIt('X'))
        ->function($this->function->dummy)->wasCalled()->once()
    ;      
  }
}