to fwd.diff :l
  if empty? :l [output []]
  if empty? bf :l [output []]
  output fput (first bf :l)-(first :l) fwd.diff bf :l
end
to nth.fwd.diff :n :l
  if :n = 0 [output :l]
  output nth.fwd.diff :n-1 fwd.diff :l
end

show nth.fwd.diff 9 [90 47 58 29 22 32 55 5 55 73]
[-2921]