pf.apple

λn.
  { ni ⟜ ⌊(√(ℝn))
  ; pns ← (⍳ 2 ni 1)
  ; isPrime ← λn.¬((∨)/ₒ #f ([(n|x)=0]'(⍳ 2 (⌊(√(ℝn))) 1))); pf ⇐ (isPrime #.)
  ; pps ⟜  pf ((λk. ((n|k)=0)) #. pns)
  -- prime factors of n
  ; (pf (n⊲(n/.)'pps))⧺pps
  }