連続な重複番号を除く

// input
val xs = (1, "aaa")::(1, "bbb")::(1, "ccc")::(2, "abc")::(3, "xxx")::(3, "kkk")::(1, "abcde")::(1, "qwee")::Nil

// output
val expect = List((1, "aaa"), (2, "abc"), (3, "xxx"), (1, "abcde"))

// fold
val resultFold = xs.foldLeft(List[(Int, String)]()){ 
               case (Nil, t) => List(t)
               case (ys@((i, s)::_), t@(j, q)) => if(i == j) ys else t::ys 
           }.reverse
assert(expect == resultFold)

// invert
val resultInvert = xs.head :: (xs, xs.tail).invert.collect{ case ((i, x), (j, y)) if i != j => (j, y) }
assert(expect == resultInvert )