F amb(comp, options, prev = ‘’) -> Array[String]
   I options.empty
      R []

   L(opt) options[0]
      // If this is the base call, prev is empty and we need to continue.
      I prev != ‘’ & !comp(prev, opt)
         L.continue

      // Take care of the case where we have no options left.
      I options.len == 1
         R [opt]

      // Traverse into the tree.
      V res = amb(comp, options[1..], opt)

      // If it was a failure, try the next one.
      if !res.empty
         R opt [+] res // We have a match

   R []

V sets = [[‘the’, ‘that’, ‘a’],
          [‘frog’, ‘elephant’, ‘thing’],
          [‘walked’, ‘treaded’, ‘grows’],
          [‘slowly’, ‘quickly’]]

V result = amb((s, t) -> s.last == t[0], sets)
print(result.join(‘ ’))