:: Project Euler #4
:: find largest palindrome made from product of two 3 digit numbers
|=  a=@
=<  (palindrome a)
::
|%
++  reverse
  |=  n=@
  =|  reversed=@
  |-  ^-  @
    ?:  (lte n 0)
      reversed
    $(n (div n 10), reversed (add (mul reversed 10) (mod n 10)))
++  is-pal
  |=  m=@
  ^-  ?
    =(m (reverse m))
++  palindrome
  |=  lrg-pal=@
  =/  a=@  999
  |-  ^-  @
  ?:  (lth a 100)
    lrg-pal
  =|  b=@
    =.  b  999
    |-  ^-  @
    ?:  (lth b a)
      ^$(a (dec a))                      :: if b<a, go to outer loop
    ?:  (lte (mul a b) lrg-pal)
      $(b (dec b))
    ?:  (is-pal (mul a b))
      $(lrg-pal (mul a b), b (dec b))
    $(b (dec b))
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