It has a much, much worse bug than the negative index: your definition of sort says "given x produce a y which is sorted and completely unrelated to x in any way". The implementation "given x return y=[1,2,3]" satisfies your specification...

The negative index issue is a valid bug concern because when I was typing out the _fictional_ PSL code I missed the negative index hence my reply that said language could be smart enough to handle that (ie, tongue in cheek). However, your bug is not legit, because you've introduced extra wording for an assumption you can't make since you're not the creator of my fictional language. For PSL, "Given SET(x), Find SET(y)" implies that SET(y) is a resultant set of SET(X). SET(y) could have a case where the elements are different than SET(x) but only where explicitly indicated by the requirements. The requirements specified by the where clause in this case do not dictate a transformation on the elements, only a re-ordering. So no, it's not a bug.

It's funny that despite it being a passing pseudo example to make a point about programs being compositions of tasks that I'd be this vested in defending the code correctness of my pseudo code but I can't help it.

Actually, I don't see why it couldn't be 'smart' enough to know how to handle non-existant array locations. It just needs to know to handle all the valid cases, and ignore the invalid ones, and it's good to go.