Help in Creating Index in DTL based of a variable value

Hello. I need to transform a message

FROM:

MSH|^~\&|
SCH||61490||
PID|1||
RGS|1||1
AIS|1||
AIS|2||
AIS|3||
AIL|1||
AIP|1||

TO:

MSH|^~\&|
SCH||61490||
PID|1||
RGS|1||1
AIS|1||
AIL|1||
AIP|1||
RGS|1||1
AIS|2||
AIL|1||
AIP|1||
RGS|1||1
AIS|3||
AIL|1||
AIP|1||

The RGS, AIS, AIL and AIP are all under the RGS group. The one RGS segment that comes in will be copied across the group. If 3 AIS segments come in then I need 3 RGS groups, if 2 I need 2 RGS groups etc.

In my DTL (screenshot below) I have currently hardcoded the RGS index (1,2,3) but this will not be sufficient incase 4 AIS segments are sent in. I need this index to be variable based off the number of AIS segments that are sent in. 

I have seen posts about using "increment and a while" and I did try that but it didn't work. In the DTL above instead of setting the index to 1, 2, 3 I had set it to "trgs". tais is the count of the AIS sent in. 

Schema:

<?xml version="1.0"?> <Category name="TEST_Schema_2.3.1" 
          description="Orders scheduling." 
          base="2.3.1">     <MessageType name="SIU_S12" structure="SIU_ALL" returntype="base:ACK_S12"/>
    <MessageType name="SIU_S13" structure="SIU_ALL" returntype="base:ACK_S13"/>
    <MessageType name="SIU_S14" structure="SIU_ALL" returntype="base:ACK_S14"/>
    <MessageType name="SIU_S15" structure="SIU_ALL" returntype="base:ACK_S15"/>     <MessageStructure name="SIU_ALL" 
          definition="base:MSH~base:SCH~base:PID~{~base:RGS~base:AIS~base:AIL~base:AIP~}" 
          description="Custom structure with repeating RGS group"/>
</Category>

Any leads please?