Task: Amb
C: typedef const char * amb_t;

amb_t amb(size_t argc, ...)
{
  amb_t *choices;
  va_list ap;
  int i;
  
  if(argc) {
    choices = malloc(argc*sizeof(amb_t));
    va_start(ap, argc);
    i = 0;
    do { choices[i] = va_arg(ap, amb_t); } while(++i < argc);
    va_end(ap);
    
    i = 0;
    do { TRY(choices[i]); } while(++i < argc);
    free(choices);
  }
  
  FAIL;
}

int joins(const char *left, const char *right) { return left[strlen(left)-1] == right[0]; }

int _main() {
  const char *w1,*w2,*w3,*w4;
  
  w1 = amb(3, "the", "that", "a");
  w2 = amb(3, "frog", "elephant", "thing");
  w3 = amb(3, "walked", "treaded", "grows");
  w4 = amb(2, "slowly", "quickly");
  
  if(!joins(w1, w2)) amb(0);
  if(!joins(w2, w3)) amb(0);
  if(!joins(w3, w4)) amb(0);
  
  printf("%s %s %s %s\n", w1, w2, w3, w4);
  
  return EXIT_SUCCESS;
}

C#: using System;
using System.Collections.Generic;

public class Amb : IDisposable
{
    List<IValueSet> streams = new List<IValueSet>();
    List<IAssertOrAction> assertsOrActions = new List<IAssertOrAction>();
    volatile bool stopped = false;

    public IAmbValue<T> DefineValues<T>(params T[] values)
    {
        return DefineValueSet(values);
    }

    public IAmbValue<T> DefineValueSet<T>(IEnumerable<T> values)
    {
        ValueSet<T> stream = new ValueSet<T>();
        stream.Enumerable = values;
        streams.Add(stream);
        return stream;
    }

    public Amb Assert(Func<bool> function)
    {
        assertsOrActions.Add(new AmbAssert()
        {
            Level = streams.Count,
            IsValidFunction = function
        });
        return this;
    }

    public Amb Perform(Action action)
    {
        assertsOrActions.Add(new AmbAction()
        {
            Level = streams.Count,
            Action = action
        });
        return this;
    }

    public void Stop()
    {
        stopped = true;
    }

    public void Dispose()
    {
        RunLevel(0, 0);
        if (!stopped)
        {
            throw new AmbException();
        }
    }

    void RunLevel(int level, int actionIndex)
    {
        while (actionIndex < assertsOrActions.Count && assertsOrActions[actionIndex].Level <= level)
        {
            if (!assertsOrActions[actionIndex].Invoke() || stopped)
                return;
            actionIndex++;
        }
        if (level < streams.Count)
        {
            using (IValueSetIterator iterator = streams[level].CreateIterator())
            {
                while (iterator.MoveNext())
                {
                    RunLevel(level + 1, actionIndex);
                }
            }
        }
    }

    interface IValueSet
    {
        IValueSetIterator CreateIterator();
    }

    interface IValueSetIterator : IDisposable
    {
        bool MoveNext();
    }

    interface IAssertOrAction
    {
        int Level { get; }
        bool Invoke();
    }

    class AmbAssert : IAssertOrAction
    {
        internal int Level;
        internal Func<bool> IsValidFunction;

        int IAssertOrAction.Level { get { return Level; } }

        bool IAssertOrAction.Invoke()
        {
            return IsValidFunction();
        }
    }

    class AmbAction : IAssertOrAction
    {
        internal int Level;
        internal Action Action;

        int IAssertOrAction.Level { get { return Level; } }

        bool IAssertOrAction.Invoke()
        {
            Action(); return true;
        }
    }

    class ValueSet<T> : IValueSet, IAmbValue<T>, IValueSetIterator
    {
        internal IEnumerable<T> Enumerable;
        private IEnumerator<T> enumerator;

        public T Value { get { return enumerator.Current; } }

        public IValueSetIterator CreateIterator()
        {
            enumerator = Enumerable.GetEnumerator();
            return this;
        }

        public bool MoveNext()
        {
            return enumerator.MoveNext();
        }

        public void Dispose()
        {
            enumerator.Dispose();
        }
    }
}

public interface IAmbValue<T>
{
    T Value { get; }
}

public class AmbException : Exception
{
    public AmbException() : base("AMB is angry") { }
}