$ include "seed7_05.s7i";

const type: setListType is array array string;

const func array string: amb (in string: word1, in setListType: listOfSets) is func
  result
    var array string: ambResult is 0 times "";
  local
    var string: word2 is "";
  begin
    for word2 range listOfSets[1] do
      if length(ambResult) = 0 and word1[length(word1) len 1] = word2[1 len 1] then
        if length(listOfSets) = 1 then
          ambResult := [] (word1) & [] (word2);
        else
          ambResult := amb(word2, listOfSets[2 ..]);
          if length(ambResult) <> 0 then
            ambResult := [] (word1) & ambResult;
          end if;
        end if;
      end if;
    end for;
  end func;

const func array string: amb (in setListType: listOfSets) is func
  result
    var array string: ambResult is 0 times "";
  local
    var string: word1 is "";
  begin
    for word1 range listOfSets[1] do
      if length(ambResult) = 0 then
        ambResult := amb(word1, listOfSets[2 ..]);
      end if;
    end for;
  end func;

const proc: main is func
  local
    var array string: ambResult is 0 times "";
    var string: word is "";
  begin
    ambResult := amb([] ([] ("the", "that", "a"),
                         [] ("frog", "elephant", "thing"),
                         [] ("walked", "treaded", "grows"),
                         [] ("slowly", "quickly")));
    for word range ambResult do
      write(word <& " ");
    end for;
    writeln;
  end func;