local utf8substr = function(str, pos1, pos2)
    if type(str)=="number" then str = tostring(str) end
    if type(str)~="string" then
        error("bad argument #1 to 'utf8substr' (string expected, got "..type(str)..")", 2)
        return str
    end
    if pos1 and type(pos1)~="number" then
        error("bad argument #2 to 'utf8substr' (number expected, got "..type(pos1)..")", 2)
        return str
    end
    if pos2 and type(pos2)~="number" then
        error("bad argument #3 to 'utf8substr' (number expected, got "..type(pos2)..")", 2)
        return str
    end
    pos1 = pos1 or 1
    pos2 = pos2 or -1
    if pos1<0 or pos2<0 then
        local len = utf8strlen(str)
        if pos1<0 then pos1 = len+pos1+1 end
        if pos2<0 then pos2 = len+pos2+1 end
    end
    local fn = rawget(GLOBAL, "utf8substr")
    if fn then pos1 = pos1-1 end --c-like numbering behaviour
    fn = fn or str.sub
    return fn(str, pos1, pos2)
end