produces the cartesian product of two sets-of-strings in Hoon

cartesian-product.hoon

:-  %say  |=  *  :-  %noun
=<  (cartesian-product (sy ['a' 'b' 'c' ~]) (sy ['x' 'y' 'z' ~]))
::
|%  ++  cartesian-product
  |=  {a/(set @t) b/(set @t)}
  =+  r=*(set {@t @t})
  |-  ?~  a  r
  =+  b=b                                    :: shadow b for inner loop walkthru
  |-  ?~  b
    =+  ^=  r
      ^$(a l.a, r r)
    ^$(a r.a, r r)
  =+  ^=  r
    (~(put in r) [n.a n.b])
  =+  ^=  r
    $(b l.b, r r)
  $(b r.b, r r)
--