I rarely find the GRT worth the bother. It is ridiculously hard to maintain GRT code, and it's not unlikely for the transformation step in a GRT to be so computionally complex that the order-by-sorted-indices method as per your second snippet beats it anyway — as seen at Re: To use a module...or not.. Order-by-sorted-indices is generally straightforward, easy to maintain, and easy to get consistent results from; yet very fast and memory efficient. I doubt I'll ever need anything else (though it's not completely impossible, of course).

I appreciate the ST as an excercise in functional thinking, but it's been a long time since I last used it in practice.