{def fac
 {lambda {:n}
  {if {< :n 1}
   then 1
   else {long_mult :n {fac {- :n 1}}}}}}

{fac 6}
-> 720

{fac 100}
-> 93326215443944152681699238856266700490715968264381621468592963895217599993229
915608941463976156518286253697920827223758251185210916864000000000000000000000000