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

-- quite some code deals with exporting references  --

-- links:
--
-- url      :
-- file     :
-- internal : automatic location
-- location : named reference

-- references:
--
-- implicit : automatic reference
-- explicit : named reference

local tonumber = tonumber
local lpegmatch = lpeg.match
local insert = table.insert

local references      = structures.references

local structurestags  = structures.tags
local specifications  = structurestags.specifications
local locatedtag      = structurestags.locatedtag

local backend         = structurestags.backend

local setattribute    = backend.setattribute
local extras          = backend.extras
local fixes           = backend.fixes
local referencehash   = backend.referencehash
local destinationhash = backend.destinationhash

local implement       = interfaces.implement

local evaluators = { }
local specials   = { }
local explicits  = { }

evaluators.inner = function(di,var)
    local inner = var.inner
    if inner then
        setattribute(di,"location",inner,true)
    end
end

evaluators.outer = function(di,var)
    local file, url = references.checkedfileorurl(var.outer,var.outer)
    if url then
        setattribute(di,"url",url,true)
    elseif file then
        setattribute(di,"file",file,true)
    end
end

evaluators["outer with inner"] = function(di,var)
    local file = references.checkedfile(var.f)
    if file then
        setattribute(di,"file",file,true)
    end
    local inner = var.inner
    if inner then
        setattribute(di,"inner",inner,true)
    end
end

evaluators.special = function(di,var)
    local handler = specials[var.special]
    if handler then
        handler(di,var)
    end
end

do

    evaluators["special outer with operation"]     = evaluators.special
    evaluators["special operation"]                = evaluators.special
    evaluators["special operation with arguments"] = evaluators.special

    function specials.url(di,var)
        local url = references.checkedurl(var.operation)
        if url and url ~= "" then
            setattribute(di,"url",url,true)
        end
    end

    function specials.file(di,var)
        local file = references.checkedfile(var.operation)
        if file and file ~= "" then
            setattribute(di,"file",file,true)
        end
    end

    function specials.fileorurl(di,var)
        local file, url = references.checkedfileorurl(var.operation,var.operation)
        if url and url ~= "" then
            setattribute(di,"url",url,true)
        elseif file and file ~= "" then
            setattribute(di,"file",file,true)
        end
    end

    function specials.internal(di,var)
        local internal = references.checkedurl(var.operation)
        if internal then
            setattribute(di,"location",internal)
        end
    end

    local function adddestination(di,references) -- todo: specials -> exporters and then concat
        if references then
            local reference = references.reference
            if reference and reference ~= "" then
                local prefix = references.prefix
                if prefix and prefix ~= "" then
                    setattribute(di,"prefix",prefix,true)
                end
                setattribute(di,"destination",reference,true)
                for i=1,#references do
                    local r = references[i]
                    local e = evaluators[r.kind]
                    if e then
                        e(di,r)
                    end
                end
            end
        end
    end

    function extras.addimplicit(di,references)
        if references then
            local internal = references.internal
            if internal then
                setattribute(di,"implicit",internal)
            end
        end
    end

    function extras.addinternal(di,references)
        if references then
            local internal = references.internal
            if internal then
                setattribute(di,"internal",internal)
            end
        end
    end

    local p_firstpart = lpeg.Cs((1-lpeg.P(","))^0)

    local function addreference(di,references)
        if references then
            local reference = references.reference
            if reference and reference ~= "" then
                local prefix = references.prefix
                if prefix and prefix ~= "" then
                    setattribute(di,"prefix",prefix)
                end
                setattribute(di,"reference",reference,true)
                setattribute(di,"explicit",lpegmatch(p_firstpart,reference),true)
            end
            local internal = references.internal
            if internal and internal ~= "" then
                setattribute(di,"implicit",internal)
            end
        end
    end

    local function link(di,element,n,fulltag)
        -- for instance in lists a link has nested elements and no own text
        local reference = referencehash[fulltag]
        if reference then
            adddestination(di,structures.references.get(reference))
            return true
        else
            local data = di.data
            if data then
                for i=1,#data do
                    local di = data[i]
                    if di then
                        local fulltag = di.fulltag
                        if fulltag and link(di,element,n,fulltag) then
                            return true
                        end
                    end
                end
            end
        end
    end

    local function reference(di,element,n,fulltag)
        local destination = destinationhash[fulltag]
        if destination then
            local d = structures.references.internals[destination]
            if d then
                addreference(di,d.references)
                return true
            else
                return false
            end
        else
            local data = di.data
            if data then
                for i=1,#data do
                    local di = data[i]
                    if di then
                        local fulltag = di.fulltag
                        if fulltag and reference(di,element,n,fulltag) then
                            return true
                        end
                    end
                end
            end
        end
    end

    extras.adddestination = adddestination
    extras.addreference   = addreference

    extras.link           = link
    extras.reference      = reference

end

do

    function fixes.linenumber(di,data,i)
        local ni = data[i+1]
        if ni then
            if ni.data then
                while true do
                    local d = ni.data[1]
                    if d then
                        local e = d.element
                        if e then
                            if e == "line" or e == "verbatimline" then
                                insert(d.data,1,di)
                                data[i] = false
                                return
                            else
                                ni = d
                            end
                        else
                            return
                        end
                    else
                        return
                    end
                end
            end
        end
    end

end