let rec fact = function
  | 0 | 1 -> 1
  | n when n < 0 -> invalid_arg "fact: arg must be positive"
  | n -> n * fact (n - 1)

let rec is_sorted = function
  | [] | [_] -> true
  | n1::(n2::_ as l) -> n1 <= n2 && is_sorted l

let safe_tail (_::l | l) = l

• Why I Love F#: Pattern Matching (Dustin Campbell)