improved let wrt .globals

@@ -907,10 +907,12 @@ class Expression (ArcAnnotation) :
         else :
             env = binding._dict
             env["__binding__"] = binding._dict
+            env["__globals__"] = self.globals
             try :
                 return Token(self.globals(self._expr, env))
             finally :
                 del env["__binding__"]
+                del env["__globals__"]
     def __call__ (self, binding) :
         """Returns the value from `bind` (but not encapsulated in a
         `Token`).

@@ -67,11 +67,9 @@ def unlet (expr, *names) :
     return unparse(new), drop.calls
 
 class MakeLet (object) :
-    def __init__ (self, globals) :
-        self.globals = globals
     def match (self, match, binding) :
         env = dict(binding)
-        env.update(iter(self.globals))
+        env.update(env["__globals__"])
         exec("", env)
         old = set(env)
         exec(match, env)
@@ -95,5 +93,5 @@ def extend (module) :
     class PetriNet (module.PetriNet) :
         def __init__ (self, name, **args) :
             module.PetriNet.__init__(self, name, **args)
-            self.globals["let"] = MakeLet(self.globals)
+            self.globals["let"] = MakeLet()
     return PetriNet, unlet