Task: Amb
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") { }
}

Python: import itertools as _itertools
 
class Amb(object):
    def __init__(self):
        self._names2values   = {}       
        self._func           = None     
        self._valueiterator  = None     
        self._funcargnames   = None     
 
    def __call__(self, arg=None):
        if hasattr(arg, '__code__'):                
            
            
            
            globls = arg.__globals__ if hasattr(arg, '__globals__') else arg.func_globals
            
            argv = arg.__code__.co_varnames[:arg.__code__.co_argcount]
            for name in argv:
                if name not in self._names2values:
                    assert name in globls, \
                           "Global name %s not found in function globals" % name
                    self._names2values[name] = globls[name]
            
            valuesets = [self._names2values[name] for name in argv]
            self._valueiterator = _itertools.product(*valuesets)
            self._func = arg
            self._funcargnames = argv
            return self
        elif arg is not None:
            
            
            
            arg = frozenset(arg)
            return arg
        else:
            
            
            
            return self._nextinsearch()
 
    def _nextinsearch(self):
        arg = self._func
        globls = arg.__globals__
        argv = self._funcargnames
        found = False
        for values in self._valueiterator:
            if arg(*values):
                
                found = True
                for n, v in zip(argv, values):
                    globls[n] = v
                break
        if not found: raise StopIteration
        return values
 
    def __iter__(self):
        return self
 
    def __next__(self):
        return self()
    next = __next__ 
 
if __name__ == '__main__':
    if True:
        amb = Amb()
 
        print("\nSmall Pythagorean triples problem:")
        x = amb(range(1,11))
        y = amb(range(1,11))
        z = amb(range(1,11))
 
        for _dummy in amb( lambda x, y, z: x*x + y*y == z*z ):
            print ('%s %s %s' % (x, y, z))
 
 
    if True:
        amb = Amb()
 
        print("\nRosetta Code Amb problem:")
        w1 = amb(["the", "that", "a"])
        w2 = amb(["frog", "elephant", "thing"])
        w3 = amb(["walked", "treaded", "grows"])
        w4 = amb(["slowly", "quickly"])
 
        for _dummy in amb( lambda w1, w2, w3, w4: \
                             w1[-1] == w2[0] and \
                             w2[-1] == w3[0] and \
                             w3[-1] == w4[0] ):
            print ('%s %s %s %s' % (w1, w2, w3, w4))
 
    if True:
        amb = Amb()
 
        print("\nAmb problem from "
            "http://www.randomhacks.net/articles/2005/10/11/amb-operator:")
        x = amb([1, 2, 3])
        y = amb([4, 5, 6])
 
        for _dummy in amb( lambda x, y: x * y != 8 ):
            print ('%s %s' % (x, y))