-- `cat` is list-concat, not string-concat. Coming from Python/JS the
-- instinct is to write `cat "a" "b"` for "ab" - but `cat` expects a
-- list of strings as its first arg and a separator as its second.
--
-- The verifier flags `cat "a" "b"` as ILO-T013 with a suggestion
-- pointing at the two canonical text-concat shapes: `fmt` for
-- arbitrary interpolation, and `+` when both operands are text.
-- Idiom 1: `fmt` builds a string from a template and values
join-fmt>t;fmt "{}{}" "foo" "bar"
-- Idiom 2: `+` on two text values concatenates them
join-plus>t;+"foo" "bar"
-- Idiom 3: actual list-concat - what `cat` is for
join-cat>t;cat ["a" "b" "c"] ","
-- run: join-fmt
-- out: foobar
-- run: join-plus
-- out: foobar
-- run: join-cat
-- out: a,b,c