environment/lib/python2.7/site-packages/IPython/parallel/controller/dependency.py
def unreachable(self, completed, failed=None):
        """return whether this dependency has become impossible."""
        if len(self) == 0:
            return False
        against = set()
        if not self.success:
            against = completed
        if failed is not None and not self.failure:
            against = against.union(failed)
        if self.all:
            return not self.isdisjoint(against)
        else:
            return self.issubset(against)
def unreachable(self, completed, failed=None):
        """return whether this dependency has become impossible."""
        if len(self) == 0:
            return False
        against = set()
        if not self.success:
            against = completed
        if failed is not None and not self.failure:
            against = against.union(failed)
        if self.all:
            return not self.isdisjoint(against)
        else:
            return self.issubset(against)
return whether this dependency has become impossible.