Tail recursion cons operator Scala
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If the last statement in a function is func(x,tailList):

        def func(x:Int):List[Int]...
        case head :: tailList => head :: func(x,tailList)

converting this function to tail recursion requires accumulator to be added as a third parameter (and to keep it clean adding a local function inside func() ).  

        insertTail(x,tailList,head::acc) 
doesn't seem to work correctly. shouldn't "acc" hold the computation in progress?

Am I missing something here to make it tail recursive work with accumulator?