F xor(a, b)
   R (a & !b) | (b & !a)

F ha(a, b)
   R (xor(a, b), a & b)

F fa(a, b, ci)
   V (s0, c0) = ha(ci, a)
   V (s1, c1) = ha(s0, b)
   R (s1, c0 | c1)

F fa4(a, b)
   V width = 4
   V ci = [0B] * width
   V co = [0B] * width
   V s = [0B] * width
   L(i) 0 .< width
      (s[i], co[i]) = fa(a[i], b[i], I i != 0 {co[i - 1]} E 0)
   R (s, co.last)

F int2bus(n, width = 4)
   R reversed(bin(n).zfill(width)).map(c -> Int(c))

F bus2int(b)
   R sum(enumerate(b).filter2((i, bit) -> bit).map2((i, bit) -> 1 << i))

V width = 4
V tot = [0B] * (width + 1)
L(a) 0 .< 2 ^ width
   L(b) 0 .< 2 ^ width
      V (ta, tlast) = fa4(int2bus(a), int2bus(b))
      L(i) 0 .< width
         tot[i] = ta[i]
      tot[width] = tlast
      assert(a + b == bus2int(tot), ‘totals don't match: #. + #. != #.’.format(a, b, String(tot)))