Function flatten

src/group.js:12–17

(groups, keys)

}

function flatten(groups, keys) {
  for (let i = 1, n = keys.length; i < n; ++i) {
    groups = groups.flatMap(g => g.pop().map(([key, value]) => [...g, key, value]));
  }
  return groups;
}

export function flatGroup(values, ...keys) {
  return flatten(groups(values, ...keys), keys);