-- rgxall1 pat s -> L t: flat first-capture-group convenience.
--
-- rgxall returns L (L t) (each match is a list of capture groups, or a
-- 1-element list of the whole match when there are no groups). For the
-- common "extract every match of a single capture group" shape, this
-- forces a `flat` or a `map (xs:L t>t;hd xs)` step. rgxall1 returns the
-- flat L t directly.
--
-- This is the canonical html-scrape shape that the html-scraper persona's
-- 5-rerun history kept paying ~30 tokens on (the `ext` helper). PR for
-- rerun6 lands this builtin.
-- 0 groups: every whole match, flat. Parallel to `rgx` no-group except
-- rgx silently returns ONLY the first match when given a captureless
-- pattern; rgxall1 returns them all.
digits>L t;rgxall1 "\d+" "a1 b22 c333"
-- 1 group: capture-1 for every match, flat. The HN-frontpage shape from
-- the html-scraper persona collapses to one call.
titles>L t;rgxall1 "<h2>([^<]+)</h2>" "<h2>Foo</h2><h2>Bar</h2><h2>Baz</h2>"
-- Empty input: empty list, no error.
none>L t;rgxall1 "\d+" "no digits here"
-- run: digits
-- out: [1, 22, 333]
-- run: titles
-- out: [Foo, Bar, Baz]
-- run: none
-- out: []