gfind utility

--[[
        This function behaves just like gmatch, but it also returns the position
    in the line of the match.

    @return iterator function which returns the start, stop, and textual match of every
            occurence of the given pattern.

    Example:
        for start, stop, match in gfind (str, pat) do
            ...
        end
]]
local function gfind (_string, pattern)
    if not _string or not pattern then
        error ("Arguments to gfind are invalid: faulty string or pattern.")
    end

    local start, stop;

    return function()
        if stop then
            stop = stop + 1
        end
        start, stop = _string:find (pattern, stop)

        return start, stop, (start ~= nil) and _string:sub (start, stop) or nil
    end
end