[TYPO3-mvc] checkbox issues

[Thomas Mammitzsch]

hi list,

i have a problem getting checkboxes checked when a form is redisplayed 
(f.e. on validation errors). Here is what i do currently:

template:

<f:for each="{categories}" as="category">
	<div class="checkbox">
		<f:form.checkbox name="frontendUser[ptferegisterCategories][]" 
value="{category.uid}"/>
	<label for="txMysqlmssqlsyncCategorie">{category.title}</label>
	</div>
</f:for>


categories is an array of category objects. For each category object i 
render a checkbox.
The selected categories go into a property ptferegisterCategories 
(ObjectStorage) of the frontendUser object.
How can i set the relevant checkboxes checked depending on the 
categories that are stored in ptferegisterCategories property of the 
frontend user? AFAIK there are only simple comparisons possible.

regards, Thomas