Task: Amb
Go: package main

import (
    "fmt"
    "sync"
)

func ambStrings(ss []string) chan []string {
    c := make(chan []string)
    go func() {
        for _, s := range ss {
            c <- []string{s}
        }
        close(c)
    }()
    return c
}

func ambChain(ss []string, cIn chan []string) chan []string {
    cOut := make(chan []string)
    go func() {
        var w sync.WaitGroup
        for chain := range cIn {
            w.Add(1)
            go func(chain []string) {
                for s1 := range ambStrings(ss) {
                    if s1[0][len(s1[0])-1] == chain[0][0] {
                        cOut <- append(s1, chain...)
                    }
                }
                w.Done()
            }(chain)
        }
        w.Wait()
        close(cOut)
    }()
    return cOut
}

func main() {
    s1 := []string{"the", "that", "a"}
    s2 := []string{"frog", "elephant", "thing"}
    s3 := []string{"walked", "treaded", "grows"}
    s4 := []string{"slowly", "quickly"}
    c := ambChain(s1, ambChain(s2, ambChain(s3, ambStrings(s4))))
    for s := range c {
        fmt.Println(s)
    }
}

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))