an interesting comment was made by HUMPHREY LYTTLETON he says,in the full sense of composing extemporarily that is without preparation,improvisation has proved to be not essential to,and practically non existent in good Jazz.
   however the statement there is improvisation in jazz is a truism so widespread that many conclude that if there is no improvisation it is not jazz.,these and the following points are made in the jazz book by Joachim E berendt
1. the once im provised is equal to improvisation.
      2.the above can be reproduced by the one who produced it, but by no one else.
      3.both improvisation and the once improvised are personal expressions of the situation of the musician who produced them. 4,
4.The concurrence of improviser, composer, and interpreter belongs to jazz improvisation.
5. insofar as the arranger corresponds to point4 , his function differs from thatof the improvising composing interpreter merely in terms ofcraftsmanship and technique;the arranger writes,even when writing for others, on the basis of his experienceas an improvising composing interpreter.
   6Improvisation-in the sense of points 1 to5- is indespensable to jazz;improvisation in the sense of complete unpreparedness and unlimited spontaneitymay occur, but it is not a necessity.
I think the most important thing here arePOINTS ONE AND TWO.if i,m playing a tune and I put in my own embellishments, Then choose to repeat it that is improvisation.
If another concertina player comes along and copies my embellishments note for note then it is not IMPROVISATION. Dick Miles