Not my specification (drive by third party)

but I do take the view that ( 1, 2, 3, 4, 5, 5, 6 ) is a list of seven values (perhaps the number of dollars in the pockets of seven distinct unique people) and when sorted the output should also have seven items that correspond to the seven input items.

> Also ...

Yeah, that needs tightening up by pastel8739

▲ Jtsummers 44 minutes ago | parent | next [-]

You need a way to differentiate the two 5s, that isn't present. If you had a list like:

  L = [(5,foo), (2,bar), (2,baz),...]

And did a:

  SORT(L, key=first) # or however it'd be specified

Then the duplicate 2s would be fine, because they're no longer duplicates, only duplicate keys. But it would still fail if (2,baz) showed up twice in the source and destination even though we've asked for SORT, not UNIQSORT.

	▲	defrost 12 minutes ago \| parent \| next [-]
		More seriously, > You need a way to differentiate the two 5s As there's no unique filtering or other reduction going on here, there's a permutation chain from input to output.
	▲	defrost 28 minutes ago \| parent \| prev [-]
		In the cases of `SORT ( 3, 2, 5, 5 ) ->> ( 2, 3, 5, 5 ) and SORT ( 3, 2, 5, 5 ) ->> ( 2, 3, 5, 5 )` one or both of those might be incorrect ? ( I'm teasing, perhaps )

▲ inigyou 32 minutes ago | parent | prev [-]

The specification said

    Every item in I is present exactly once in S

5 is an item in I, and it is present exactly once in S.

▲

defrost 26 minutes ago | parent [-]

and 5 is another item in I, and it's not present in S.

▲

inigyou 22 minutes ago | parent [-]

Yes it is, it's right there, between the 4 and the 6.

	▲	defrost 11 minutes ago \| parent [-]
		That's not the same one - track the permutation chain.