start -> part(s)    { start.t := join ('', part(s).t) }

part -> AnB         { part.t := AnB.t }
part -> 'a'         { part.t := 'a' }
part -> /[^a]+/     { part.t := /[^a]+/.t }

AnB -> 'a' AnB 'b'  { AnB.t := 'c' . AnB.t . 'd' }
AnB -> 'ab'         { AnB.t := 'cd' }