let bag,newt,id = aux bag newpos newctx id t in
if post = [] then bag, pre@[newt], [], id
else bag, pre @ [newt], List.tl post, id)
let bag,newt,id = aux bag newpos newctx id t in
if post = [] then bag, pre@[newt], [], id
else bag, pre @ [newt], List.tl post, id)