[NTG-context] Xml filtering in Lua

[mf]

This works:

local metadata = xml.filter (t, '../../topics/topic[@t:id=="' .. ch_id 
.. '"]')

also this:

local lpath = string.format('../../topics/topic[@t:id=="%s"]', ch_id)

local metadata = xml.filter (t, lpath)


It looks like xml.filter supports only 2 arguments (see lxml-tex.lua), 
and so it doesn't let you use string formatting patterns like the 
"context" command does.

You can write:

context('the value of @t:id is "%s"', ch_id)

but you can't write:

xml.filter (t, '../../topics/topic[@t:id=="%s"]', ch_id)


Best wishes,

Massi