Document(

  Prefix(pred <http://www.w3.org/2007/rif-builtin-predicate#>)
  Prefix(func <http://www.w3.org/2007/rif-builtin-function#>)
  Prefix(ex   <http://example.org/example#>)

  Group
  (
    ex:factorial(0) = 1

    Forall ?N (
      ex:factorial(?N) = External(func:numeric-multiply(
             ?N
             ex:factorial(External(func:numeric-subtract(?N 1)))))
        :- External(pred:numeric-greater-than(?N 0))
    )
  )
)