How to use the b2.util.set function in b2

# remove elements of 'bypassed' that are in 'consumed'
        
        # Suppose the target type of current generator, X is produced from 
        # X_1 and X_2, which are produced from Y by one generator.
        # When creating X_1 from Y, X_2 will be added to 'bypassed'
        # Likewise, when creating X_2 from Y, X_1 will be added to 'bypassed'
        # But they are also in 'consumed'. We have to remove them from
        # bypassed, so that generators up the call stack don't try to convert
        # them. 

        # In this particular case, X_1 instance in 'consumed' and X_1 instance
        # in 'bypassed' will be the same: because they have the same source and
        # action name, and 'virtual-target.register' won't allow two different
        # instances. Therefore, it's OK to use 'set.difference'.
        
        bypassed = set.difference(bypassed, consumed)

        return (consumed, bypassed)

grist, and sub-property values will be expressed as elements joined
        to the corresponding main property.
    """
    if __debug__:
        from .property import Property
        assert is_iterable_typed(properties, Property)
    # remove properties implied by composite features
    components = []
    component_features = set()
    for property in properties:
        if property in __composite_properties:
            cs = __composite_properties[property]
            components.extend(cs)
            component_features.update(c.feature for c in cs)

    properties = b2.util.set.difference (properties, components)

    # handle subfeatures and implicit features

    # move subfeatures to the end of the list
    properties = [p for p in properties if not p.feature.subfeature] +\
        [p for p in properties if p.feature.subfeature]

    result = []
    while properties:
        p = properties[0]
        f = p.feature

        # locate all subproperties of $(x[1]) in the property set
        subproperties = [x for x in properties if is_subfeature_of(p, x.feature)]

        if subproperties:

if len (self.alternatives_) == 1:
            return self.alternatives_ [0]

        if debug:
            print "Property set for selection:", property_set

        for v in self.alternatives_:
            properties = v.match (property_set, debug)
                       
            if properties is not None:
                if not best:
                    best = v
                    best_properties = properties

                else:
                    if b2.util.set.equal (properties, best_properties):
                        return None

                    elif b2.util.set.contains (properties, best_properties):
                        # Do nothing, this alternative is worse
                        pass

                    elif b2.util.set.contains (best_properties, properties):
                        best = v
                        best_properties = properties

                    else:
                        return None

        return best

result = []
    while properties:
        p = properties[0]
        f = p.feature()
        
        # locate all subproperties of $(x[1]) in the property set
        subproperties = __select_subproperties (p, properties)
        
        if subproperties:
            # reconstitute the joined property name
            subproperties.sort ()
            joined = b2.build.property.Property(p.feature(), p.value() + '-' + '-'.join ([sp.value() for sp in subproperties]))
            result.append(joined)

            properties = b2.util.set.difference(properties[1:], subproperties)

        else:
            # eliminate properties whose value is equal to feature's
            # default and which are not symmetric and which do not
            # contradict values implied by composite properties.
            
            # since all component properties of composites in the set
            # have been eliminated, any remaining property whose
            # feature is the same as a component of a composite in the
            # set must have a non-redundant value.
            if p.value() != f.default() or f.symmetric():
                result.append (p)
                  #\
                   #or get_grist (fullp) in get_grist (components):
                   # FIXME: restore above

def try_one_generator (project, name, generator, target_type, properties, sources):
    """ Checks if generator invocation can be pruned, because it's guaranteed
        to fail. If so, quickly returns empty list. Otherwise, calls
        try_one_generator_really.
    """
    source_types = []

    for s in sources:
        source_types.append (s.type ())

    viable_source_types = viable_source_types_for_generator (generator)
    
    if source_types and viable_source_types != ['*'] and\
           not set.intersection (source_types, viable_source_types):
        if project.manager ().logger ().on ():
            id = generator.id ()            
            project.manager ().logger ().log (__name__, "generator '%s' pruned" % id)
            project.manager ().logger ().log (__name__, "source_types" '%s' % source_types)
            project.manager ().logger ().log (__name__, "viable_source_types '%s'" % viable_source_types)
        
        return []

    else:
        return try_one_generator_really (project, name, generator, target_type, properties, sources)

def inherit_flags(toolset, base, prohibited_properties = []):
    """Brings all flag definitions from the 'base' toolset into the 'toolset'
    toolset. Flag definitions whose conditions make use of properties in
    'prohibited-properties' are ignored. Don't confuse property and feature, for
    example on and off, so blocking one of them does
    not block the other one.
    
    The flag conditions are not altered at all, so if a condition includes a name,
    or version of a base toolset, it won't ever match the inheriting toolset. When
    such flag settings must be inherited, define a rule in base toolset module and
    call it as needed."""
    for f in __module_flags.get(base, []):
        
        if not f.condition or b2.util.set.difference(f.condition, prohibited_properties):
            match = __re_first_group.match(f.rule)
            rule_ = None
            if match:
                rule_ = match.group(1)

            new_rule_or_module = ''

            if rule_:
                new_rule_or_module = toolset + '.' + rule_
            else:
                new_rule_or_module = toolset

            __add_flag (new_rule_or_module, f.variable_name, f.condition, f.values)

cs2 = virtual_target.creating_subvariant ()
            cmt1 = cs1.main_target ()
            cmt2 = cs2.main_target ()

            action1 = self.actual_ [actual_name].action ()
            action2 = virtual_target.action ()

            properties_added = []
            properties_removed = []
            if action1 and action2:
                p1 = action1.properties ()
                p1 = p1.raw ()
                p2 = action2.properties ()
                p2 = p2.raw ()

                properties_removed = set.difference (p1, p2)
                if not properties_removed: properties_removed = "none"

                properties_added = set.difference (p2, p1)
                if not properties_added: properties_added = "none"

            # FIXME: Revive printing of real location.
            get_manager().errors()(
                "Duplicate name of actual target: '%s'\n"
                "previous virtual target '%s'\n"
                "created from '%s'\n"
                "another virtual target '%s'\n"
                "created from '%s'\n"
                "added properties: '%s'\n"
                "removed properties: '%s'\n"
                % (actual_name,
                   self.actual_ [actual_name], "loc", #cmt1.location (),

self.cache_ [signature] = []

        for t in self.cache_ [signature]:
            a1 = t.action ()
            a2 = target.action ()

            # TODO: why are we checking for not result?
            if not result:
                if not a1 and not a2:
                    result = t
                else:
                    if a1 and a2 and a1.action_name () == a2.action_name () and a1.sources () == a2.sources ():
                        ps1 = a1.properties ()
                        ps2 = a2.properties ()
                        p1 = ps1.base () + ps1.free () +\
                            b2.util.set.difference(ps1.dependency(), ps1.incidental())
                        p2 = ps2.base () + ps2.free () +\
                            b2.util.set.difference(ps2.dependency(), ps2.incidental())
                        if p1 == p2:
                            result = t

        if not result:
            self.cache_ [signature].append (target)
            result = target

        # TODO: Don't append if we found pre-existing target?
        self.recent_targets_.append(result)
        self.all_targets_.append(result)

        return result

def take(attributes, properties):
    """Returns a property set which include all
    properties in 'properties' that have any of 'attributes'."""
    result = []
    for e in properties:
        if b2.util.set.intersection(attributes, feature.attributes(get_grist(e))):
            result.append(e)
    return result

properties = v.match (property_set_, debug)

            if properties is not None:
                if not best:
                    best = v
                    best_properties = properties

                else:
                    if b2.util.set.equal (properties, best_properties):
                        return None

                    elif b2.util.set.contains (properties, best_properties):
                        # Do nothing, this alternative is worse
                        pass

                    elif b2.util.set.contains (best_properties, properties):
                        best = v
                        best_properties = properties

                    else:
                        return None

        return best