Without this change `txn GroupIndex` can inappropriately return `-1` despite all fields of `tx` are the same as all fields of `gtxns[x]`. Apparently, the reason is that either `tx` or `gtxns` can be overwritten at some point by the same content but stored in different memory location (`indexOf` uses `===` which compares objects *by reference*).

I had this problem when providing liquidity twice in a row in Pact smart contract (https://github.com/pactfi/algorand-testbed/blob/master/constant_product.teal)