What is the ellipsis (empty string) used for in a Treetop(PEG) grammar?

Question

The Treetop website gives the following explanation that I don't understand

Ellipsis An empty string matches at any position and consumes no input. It's useful when you wish to treat a single symbol as part of a sequence, for example when an alternate rule will be processed using shared code.

rule alts ( foo bar / baz '' ) { def value elements.map{|e| e.text_value } end } end

when is useful to treat a symbol as a part of sequence? Can anybody provide a meaningful example of that?

Answer 1

I am not familiar with Treetop. From the example it would seem that ( foo bar / baz '' ) would either produce ['foo', 'bar'] or ['baz', ''].

If you remove the ellipsis, you would get either ['foo', 'bar'] or just 'baz' (no sequence/list/array).