s@(Sequence traits) forwardDifference
[
  s allButFirst with: s allButLast collect: #- `er
].

s@(Sequence traits) forwardDifference
"Without creating two intermediate throwaway Sequences."
[
  result ::= s allButFirst.
  result doWithIndex: [| :nextValue :index | result at: index infect: [| :oldValue | oldValue - (s at: index)].
  result
].

s@(Sequence traits) forwardDifference: n
[
  (0 below: n) inject: s into: [| :seq :_ | seq forwardDifference]
].