if not modules then modules = { } end modules ['typo-stc'] = {
    version   = 1.001,
    comment   = "companion to typo-stc.mkxl",
    author    = "Hans Hagen, PRAGMA-ADE, Hasselt NL",
    copyright = "PRAGMA ADE / ConTeXt Development Team",
    license   = "see context related readme files"
}

local nodes, node = nodes, node

local nuts       = nodes.nuts
local tonut      = nuts.tonut
local getattr    = nuts.getattr
----- getwhd     = nuts.getwhd
local getboth    = nuts.getboth
local setlink    = nuts.setlink
local nextlist   = nuts.traversers.list

local a_swapping <const> = attributes.private("swapping")

local swapping       = typesetters.swapping or { }
typesetters.swapping = swapping

local enabled = false
local stack   = { }

local report  = logs.reporter("swapping")

interfaces.implement {
    name      = "enableswapping",
    actions   = function()
        if not enabled then
            nodes.tasks.enableaction("shipouts", "typesetters.swapping.handler")
            enabled = true
        end
    end,
}

-- check if wd/ht/dp match

local function process(head, parent)
    for current, id, subtype, list in nextlist, head do
        if list then
            local a = getattr(current,a_swapping)
            if a then
                if not stack then
                    stack = { }
                end
                local s = stack[a]
                if not s then
                    s = { }
                    stack[a] = s
                end
             -- s[#s+1] = { parent, current }
                s[#s+1] = current
            end
            process(list,current)
        end
    end
    return head
end

swapping.handler = function(head)
    if enabled then
        stack = { }
        head  = process(head)
        if stack then
            for k, v in next, stack do
                local parent  = v[1]
                local box     = v[2]
                local columns = k & 0xFFFF
                local w = lua.newindex(#v)
                local rows = #v//columns
                local vn = 1
                local wr = 0
                local wc = 1
                for r=1,rows do
                    for c=1,columns do
                        wr = wr + 1
                        if wr > rows then
                            wc = wc + 1
                            wr = 1
                        end
                        local wn = (wr-1)*columns+wc
                     -- local old = v[vn]
                     -- local new = v[wn]
                     -- local vi  = old[2]
                     -- local prv, nxt = getboth(new[2])
                        --
                     -- local oldwd, oldht, olddp = getwhd(old[1])
                     -- local newwd, newht, newdp = getwhd(new[1])
                     -- if oldht ~= newht or olddp ~= newdp then
                     --     report("difference old (%p,%p), new (%p,%p)",oldht,olddp,newht,newdp)
                     -- end
                        local vi  = v[vn]
                        local prv, nxt = getboth(v[wn])
                        --
                        w[wn] = { vi, prv, nxt }
                        vn = vn + 1
                    end
                end
                for i=1,#w do
                    local wi = w[i]
                    local vi = wi[1]
                    if vi and wi and vi ~= wi then
                        setlink(wi[2],vi,wi[3])
                    end
                end
            end
        end
        stack = false
    end
    return head
end