Examples:>>> result = [[('a', 1), ('b', 2)],...           [('a', 1), ('b', 3)],...           [('a', 2), ('b', 3)]]>>> problem = Problem(BacktrackingSolver())>>> problem.addVariables(["a", "b"], [1, 2, 3])>>> problem.addConstraint(lambda a, b: b > a, ["a", "b"])>>> solution = problem.getSolution()>>> sorted(solution.items()) in resultTrue>>> for solution in problem.getSolutionIter():...     sorted(solution.items()) in resultTrueTrueTrue>>> for solution in problem.getSolutions():...     sorted(solution.items()) in resultTrueTrueTrue"""def __init__(self, forwardcheck=True):    """    @param forwardcheck: If false forward checking will not be requested                         to constraints while looking for solutions                         (default is true)    @type  forwardcheck: bool    """    self._forwardcheck = forwardcheckdef getSolutionIter(self, domains, constraints, vconstraints, lst):    forwardcheck = self._forwardcheck    assignments = {}    queue = []    while True:        # Mix the Degree and Minimum Remaing Values (MRV) heuristics        for variable in lst:            if variable not in assignments:                # Found unassigned variable                values = domains[variable][:]                if forwardcheck:                    pushdomains = [                        domains[x]                        for x in domains                        if x not in assignments and x != variable                    ]                else:                    pushdomains = None                break        else:            # No unassigned variables. We've got a solution. Go back            # to last variable, if there's one.            yield assignments.copy()            if not queue:                return            variable, values, pushdomains = queue.pop()            if pushdomains:                for domain in pushdomains:                    domain.popState()        while True:            # We have a variable. Do we have any values left?            if not values:                # No. Go back to last variable, if there's one.                del assignments[variable]                while queue:                    variable, values, pushdomains = queue.pop()                    if pushdomains:                        for domain in pushdomains:                            domain.popState()                    if values:                        break                    del assignments[variable]                else:                    return            # Got a value. Check it.            assignments[variable] = values.pop()            if pushdomains:                for domain in pushdomains:                    domain.pushState()            for constraint, variables in vconstraints[variable]:                if not constraint(variables, domains, assignments, pushdomains):                    # Value is not good.                    break            else:                break            if pushdomains:                for domain in pushdomains:                    domain.popState()        # Push state before looking for next variable.        queue.append((variable, values, pushdomains))    raise RuntimeError("Can't happen")def getSolutions(self, domains, constraints, vconstraints):    lst = [        (-len(vconstraints[variable]), len(domains[variable]), variable)        for variable in domains    ]    lst.sort()    return list(self.getSolutionIter(domains, constraints, vconstraints, [c for a,b,c in lst]))