# collections.sort-by

The sort is stable: records that tie on the key come out in the order they went
in, in BOTH directions. Descending negates the comparison rather than reversing
the list, so the tied block is not silently flipped - that is what makes sorting
by one column and then another compose into the multi-column sort a user
expects.

The total order for mixed values is: booleans, then numbers, then strings, and
absent values last. Absent values sink to the bottom in both directions, because
'the rows we know nothing about' belong at the end of a descending table as much
as an ascending one; reversing them would put the empty rows first, which no one
has ever wanted.

A missing key and a null value are the same thing and both sort last.

Strings compare by Unicode code point, spelled out rather than inherited:
JavaScript's < compares UTF-16 code units and disagrees with Python and Rust
above U+FFFF. Comparison is case-sensitive, so 'Zebra' sorts before 'apple';
lowercase the key first if you want a case-insensitive sort.

A list or a map at the sort key is an error, and it is detected in one pass
before sorting starts. Discovering it mid-comparison would make the error depend
on which comparisons that language's sort happened to perform.