fun toHex 10 = "A"  
  | toHex 11 = "B"  
  | toHex 12 = "C"
  | toHex 13 = "D"  
  | toHex 14 = "E"  
  | toHex 15 = "F"
  | toHex n  = if (n < 10) then
			   Int.toString n
		       else
			   toHex(n div 16) ^ toHex(n mod 16);