fcn forwardDiff(lst){
   if(lst.len()<2)
      return(T);
      return(T(lst[1]-lst[0]).extend(forwardDiff(lst[1,*])))
}
fcn nthForwardDiff(n,xs){
   if(n==0)
      return(xs);
      return(nthForwardDiff(n-1,forwardDiff(xs))) // tail recursion
}