list - Function always return Nil -

i trying resolve anagrams assignments. , can't figure out problem behind getting list() when running sentenceanagrams function. !

type word = string type sentence = list[word] type occurrences = list[(char, int)]     def combinations(occurrences: occurrences): list[occurrences] = occurrences match {   case nil => list(nil)   case x :: xs => (for {z <- combinations(xs); <- 1 x._2} yield (x._1, i) :: z).union(combinations(xs)) }    def subtract(x: occurrences, y: occurrences): occurrences = { if (y.isempty) x else {   val ymap = y.tomap withdefaultvalue 0   x.foldleft(x) { (z, i) => if (combinations(x).contains(y)) {     val diff = i._2 - ymap.apply(i._1)     if (diff > 0) z.tomap.updated(i._1, diff).tolist else z.tomap.-(i._1).tolist   } else z   } }} 

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 def sentenceanagrams(sentence: sentence): list[sentence] = {      def sentenceanag(occ: occurrences): list[sentence] =        if (occ.isempty) list(list())       else (for {         comb <- combinations(occ)         word <- (dictionarybyoccurrences withdefaultvalue list()).apply(comb)         othersentence <- sentenceanag(subtract(occ, comb))       } yield word :: othersentence).tolist      sentenceanag(sentenceoccurrences(sentence))   }