added a plugin to provide places boundedness

+"""A plugin to implement boundedness on places.
+
+When this plugin is loaded, `Place` constructor accepts a parameter
+`bound` that allows to specify the minimal and maximal number of
+tokens that the place is allowed to carry. (Repeated tokens count as
+many times as they are repeated.) Exception `ConstraintError` is
+raised whenever an operation (like a transition firing) leads to
+violate any of a place bound. (But note that direct modifications of a
+`Place.tokens` are not checked.)
+
+>>> import snakes.plugins
+>>> snakes.plugins.load('bound', 'snakes.nets', 'nets')
+<module ...>
+
+If parameter `bound` is given as a non-negative integer, this is the
+upper bound of the place (and its lower bound is zero).
+
+>>> from nets import *
+>>> n = PetriNet('N')
+>>> p = Place('p', [dot], bound=3)
+>>> n.add_place(p)
+>>> put = Transition('put')
+>>> n.add_transition(put)
+>>> n.add_output('p', 'put', Value(dot))
+>>> p.tokens
+MultiSet([dot])
+>>> put.fire(Substitution())
+>>> p.tokens
+MultiSet([dot, dot])
+>>> put.fire(Substitution())
+>>> p.tokens
+MultiSet([dot, dot, dot])
+>>> put.fire(Substitution())
+Traceback (most recent call last):
+  ...
+ConstraintError: upper bound of place 'p' reached
+
+If `bound` is given as a pair `(n, None)` where `n` is a non-negative
+integer, then `n` is the lower bound of the place (and it has no upper
+bound).
+
+>>> n = PetriNet('N')
+>>> p = Place('p', [dot, dot], bound=(1, None))
+>>> n.add_place(p)
+>>> put = Transition('put')
+>>> n.add_transition(put)
+>>> n.add_output('p', 'put', Value(dot))
+>>> get =  Transition('get')
+>>> n.add_transition(get)
+>>> n.add_input('p', 'get', Value(dot))
+>>> p.tokens
+MultiSet([dot, dot])
+>>> get.fire(Substitution())
+>>> p.tokens
+MultiSet([dot])
+>>> get.fire(Substitution())
+Traceback (most recent call last):
+  ...
+ConstraintError: lower bound of place 'p' reached
+>>> for i in range(100) : # no upper bound
+...     put.fire(Substitution())
+
+If `bound` is given as a pair of non-negative integers `(n,m)` such
+that `n <= m` then `n` is the lower bound of the place and `m` its
+upper bound.
+
+>>> n = PetriNet('N')
+>>> p = Place('p', [dot, dot], bound=(1, 3))
+>>> n.add_place(p)
+>>> put = Transition('put')
+>>> n.add_transition(put)
+>>> n.add_output('p', 'put', Value(dot))
+>>> get =  Transition('get')
+>>> n.add_transition(get)
+>>> n.add_input('p', 'get', Value(dot))
+>>> p.tokens
+MultiSet([dot, dot])
+>>> put.fire(Substitution())
+>>> p.tokens
+MultiSet([dot, dot, dot])
+>>> put.fire(Substitution())
+Traceback (most recent call last):
+  ...
+ConstraintError: upper bound of place 'p' reached
+>>> get.fire(Substitution())
+>>> p.tokens
+MultiSet([dot, dot])
+>>> get.fire(Substitution())
+>>> p.tokens
+MultiSet([dot])
+>>> get.fire(Substitution())
+Traceback (most recent call last):
+  ...
+ConstraintError: lower bound of place 'p' reached
+
+Any other value for bound is refused raising a `ValueError`.
+
+"""
+
+from snakes.data import iterate
+from snakes import ConstraintError
+import snakes.plugins
+
+@snakes.plugins.plugin("snakes.nets")
+def extend (module) :
+    "Extends `module`"
+    class Place (module.Place) :
+        "Extend places with boundedness"
+        def __init__ (self, name, tokens, check=None, **args) :
+            """Add new keyword argument `bound`
+
+            @param args: plugin options
+            @keyword bound: the place boundaries,
+            @type bound: `int` or `(int, None)` or `(int, int)`
+            """
+            bound = args.pop("bound", (0, None))
+            if isinstance(bound, int) and bound >= 0 :
+                self._bound_min, self._bound_max = 0, bound
+            elif (isinstance(bound, tuple) and len(bound) == 2
+                  and isinstance(bound[0], int) and bound[0] >= 0
+                  and ((isinstance(bound[1], int) and bound[1] >= bound[0])
+                       or bound[1] is None)) :
+                self._bound_min, self._bound_max = bound
+            else :
+                raise ValueError("invalid value for parameter 'bound'")
+            module.Place.__init__(self, name, tokens, check, **args)
+        def add (self, tokens) :
+            """Add tokens to the place.
+
+            @param tokens: a collection of tokens to be added to the
+                place, note that `str` are not considered as iterable and
+                used a a single value instead of as a collection
+            @type tokens: `collection`
+            """
+            if (self._bound_max is not None
+                and len(self.tokens) + len(list(iterate(tokens))) > self._bound_max) :
+                raise ConstraintError("upper bound of place %r reached" % self.name)
+            module.Place.add(self, tokens)
+        def remove (self, tokens) :
+            """Remove tokens from the place.
+
+            @param tokens: a collection of tokens to be removed from the
+                place, note that `str` are not considered as iterable and
+                used a a single value instead of as a collection
+            @type tokens: `collection`
+            """
+            if len(self.tokens) - len(list(iterate(tokens))) < self._bound_min :
+                raise ConstraintError("lower bound of place %r reached" % self.name)
+            module.Place.remove(self, tokens)
+        def reset (self, tokens) :
+            """Replace the marking with `tokens`.
+
+            @param tokens: a collection of tokens to be removed from the
+                place, note that `str` are not considered as iterable and
+                used a a single value instead of as a collection
+            @type tokens: `collection`
+            """
+            count = len(list(iterate(tokens)))
+            bmax = count if self._bound_max is None else self._bound_max
+            if not (self._bound_min <= count <= bmax) :
+                raise ConstraintError("not within bounds of place %r" % self.name)
+            module.Place.reset(self, tokens)
+    return Place