concatenate string in OIC

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Hi,

   I need to concatenate string from the collection using for each loop and assign it to a variable. I have attached the flow in invoice loop.png. 

I have created a variable called dealID in Assign activity.  In the expression builder, i cant use for-each loop. So i assigned empty string.

Then i have added for each loop action pointing to the array (collection ) top most repeating element , but then i am not sure where to use this concat function to assign the strings to the dealID variable created earlier

I tried to put Assign within for each, but then its creating a new variable and i am not sure where i can traverse the for loop and use concat function. Please guide me in the OIC mapping Ui. i have not used any custom xsl