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;
}

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))