added plugin modules

+"""A plugin to record for each not to which modules it belongs.
+
+Modules are arbitrary values, each `PetriNet`, `Place` or transition
+is attached a set of modules. These modules can be given at
+construction time, as a single value or a collection of values.
+Moreover, when nodes are attached to a net, they inherit the modules
+from this net. When nodes are merged, their sets of modules is added
+to build the set of modules of the new node.
+
+Internally, modules are stored in a label called `"modules"` (see
+plugin `snakes.plugins.labels`).
+
+>>> import snakes.plugins
+>>> snakes.plugins.load(["ops", "modules"], "snakes.nets", "snk")
+<module 'snk' ...>
+>>> from snk import *
+>>> n1 = PetriNet("n1", modules="hello")
+>>> n1.add_place(Place("p"))
+>>> n1.place("p").modules()
+set(['hello'])
+>>> n1.add_place(Place("q"))
+>>> n2 = PetriNet("n2", modules="world")
+>>> n2.add_place(Place("r"))
+>>> n2.add_place(Place("s", modules="spam"))
+>>> n = n1|n2
+>>> list(sorted((p.name, p.modules()) for p in n.place()))
+[('[p|]', set(['hello'])),
+ ('[q|]', set(['hello'])),
+ ('[|r]', set(['world'])),
+ ('[|s]', set(['world', 'spam']))]
+"""
+
+from snakes.plugins import plugin, new_instance
+from snakes.data import iterate
+from snakes.pnml import Tree
+
+@plugin("snakes.nets",
+        depends=["snakes.plugins.labels"])
+def extend (module) :
+    class Transition (module.Transition) :
+        def __init__ (self, name, guard=None, **options) :
+            mod = set(iterate(options.pop("modules", [])))
+            module.Transition.__init__(self, name, guard, **options)
+            self.modules(mod)
+        def modules (self, modules=None) :
+            if modules is None :
+                return self.label("modules")
+            else :
+                self.label(modules=set(iterate(modules)))
+    class Place (module.Place) :
+        def __init__ (self, name, tokens=[], check=None, **options) :
+            mod = set(iterate(options.pop("modules", [])))
+            module.Place.__init__(self, name, tokens, check, **options)
+            self.modules(mod)
+        def modules (self, modules=None) :
+            if modules is None :
+                return self.label("modules")
+            else :
+                self.label(modules=set(iterate(modules)))
+    class PetriNet (module.PetriNet) :
+        def __init__ (self, name, **options) :
+            mod = set(iterate(options.pop("modules", [])))
+            module.PetriNet.__init__(self, name, **options)
+            self.modules(mod)
+        def modules (self, modules=None) :
+            if modules is None :
+                return self.label("modules")
+            else :
+                self.label(modules=set(iterate(modules)))
+        def add_place (self, place, **options) :
+            mod = set(iterate(options.pop("modules", self.modules())))
+            module.PetriNet.add_place(self, place, **options)
+            place.modules(place.modules() | mod)
+        def add_transition (self, trans, **options) :
+            mod = set(iterate(options.pop("modules", self.modules())))
+            module.PetriNet.add_transition(self, trans, **options)
+            trans.modules(trans.modules() | mod)
+        def merge_places (self, target, sources, **options) :
+            mod = set(iterate(options.pop("modules", self.modules())))
+            module.PetriNet.merge_places(self, target, sources, **options)
+            new = self.place(target)
+            new.modules(reduce(set.__or__,
+                               (self.place(p).lmodules()
+                                for p in sources),
+                               mod))
+        def merge_transitions (self, target, sources, **options) :
+            mod = set(iterate(options.pop("modules", self.modules())))
+            module.PetriNet.merge_transitions(self, target, sources, **options)
+            new = self.transition(target)
+            new.modules(reduce(set.__or__,
+                               (self.place(p).modules()
+                                for p in sources),
+                               mod))
+    return Transition, Place, PetriNet