Add a ContextSet.

This is not bug free yet, but it's going to be a good abstraction for a lot of small things.

jedi/common/context.py

@@ -0,0 +1,58 @@
+class ContextSet(object):
+    def __init__(self, *args):
+        self._set = set(args)
+
+    @classmethod
+    def from_iterable(cls, iterable):
+        return cls.from_set(set(iterable))
+
+    @classmethod
+    def from_set(cls, set_):
+        self = cls()
+        self._set = set_
+        return self
+
+    @classmethod
+    def from_sets(cls, sets):
+        """
+        Used to work with an iterable of set.
+        """
+        aggregated = set()
+        for set_ in sets:
+            print(set_)
+            if isinstance(set_, ContextSet):
+                aggregated |= set_._set
+            else:
+                aggregated |= set_
+        return cls.from_set(aggregated)
+
+    def __or__(self, other):
+        return ContextSet.from_set(self._set | other._set)
+
+    def __iter__(self):
+        for element in self._set:
+            yield element
+
+    def __repr__(self):
+        return '%s(%s)' % (self.__class__.__name__, ', '.join(str(s) for s in self._set))
+
+    def filter(self, filter_func):
+        return ContextSet(filter(filter_func, self._set))
+
+    def __getattr__(self, name):
+        def mapper(*args, **kwargs):
+            return ContextSet.from_sets(
+                getattr(context, name)(*args, **kwargs)
+                for context in self._set
+            )
+        return mapper
+
+
+def iterator_to_context_set(func):
+    def wrapper(*args, **kwargs):
+        return ContextSet.from_iterable(func(*args, **kwargs))
+
+    return wrapper
+
+
+NO_CONTEXTS = ContextSet()