def delete(self, doc):
        if doc.id is None:
            raise DatabaseException('Document does not have id')
        if doc.sid is None:
            try:
                rs = self.db.docs.remove({'id': doc.id}, safe=True)
                assert rs['n'] == 1
            except Exception as e:
                raise DatabaseException(e)
            return None
        doc.data = None
        return self.update(doc)
def setRT(self, r, t):
        self.r = r
        self.t = t
        x = self.r * math.cos(self.t)
        y = self.r * math.sin(self.t)
        self.Tail.Eq(self.p1)
        self.Head.setPoint(self.Tail.x + x,self.Tail.y + y)