let rec oneLineAux :int -> string = function
|0 -> ""
|n -> " " + oneLineAux (n-1)

let rec indentAux n i = (oneLineAux n) + (indentAux n (i-1))

let rec toStringAux num (P(n,s,yob,c)) i = printfn "%s%s" (indentAux num i) (List.foldBack(fun x list -> (toStringAux num x (i+1)) + list) c "")