fac f. Y NB. Factorial...
'1:`(] * (([ 128!:2 ,&<) <:))@.(0 < ])&>/'&([ 128!:2 ,&<)

   fac f.   NB. Factorial step...
1:`(] * (([ 128!:2 ,&<) <:))@.(0 < ])

   fib f. Y NB. Fibonacci...
'(([ ([ 128!:2 ,&<) ] - 2:) + [ ([ 128!:2 ,&<) ] - 1:)^:(1 < ])&>/'&([ 128!:2 ,&<)

   fib f.   NB. Fibonacci step...
(([ ([ 128!:2 ,&<) ] - 2:) + [ ([ 128!:2 ,&<) ] - 1:)^:(1 < ])