Union and Intersection of intervals

I have made the intersection function which takes intervals and unions them together. I am now working on the union function. I initially used recursion to do the job but that was incorrect that the better method was just to sort the intervals list and then use a simple loop to evaluate the union. Here is the issue i was facing:

[[A, B, C, D], E]
AUE-> False, BUE-> False, CUE-> True => [F, A, B, D]
union [F, A, B, D]-> FUA-> False => [[F,A], B, D]
again [F,A], B => FUB-> False, BUA-> False => [F, A, B] => union this list now => FUA-> False => [[F,A], B]
then again [F,A], B => FUB-> False, BUA-> False => [F, A, B] => FUA-> False => [[F,A], B]
this goes on and on infinitely
This is a problem of infinite recursion