Retrieve current input file line in Lua filter

[unitscan]

From https://pandoc.org/lua-filters:

function Header(el)
    if el.level == 1 then
      return el:walk {
        Str = function(el)
            return pandoc.Str(text.upper(el.text))
        end
      }
    end
end

It is possible to know the number(s) of the line(s) of the input file at which el.level == 1?

[jgm]

This is not generally stored in the AST. Exception: when the sourcepos extension is used
https://pandoc.org/MANUAL.html#extension-sourcepos
but it is only available for commonmark and variants (gfm).