::  Find the sum of all the multiples of 3 or 5 below n (NOT including n).
::
|=  n=@ud
=/  i=@  0
=/  sum=@  0
^-  @ud
|-
:: ~&  [i sum]  :: for debugging
?:  =(i n)
  sum
?:  ?|(=(0 (mod i 3)) =(0 (mod i 5)))
  $(i +(i), sum (add i sum))
$(i +(i))