local function getSubtag(p, f, n, i)
    p = toCfg(p)
    n = n or 1
    for i = i or 1, #p do
        if match(p[i], f) then
            n = n-1
            if n <= 0 then
                return p[i], i
            end
        end
    end
end

function lp8.children(p, f, i)
    return function(s)
        local c, i = getSubtag(p, f, 1, s.i)
        s.i = i
        return (c or et)[2], i
    end, {i = i or 1}
end