code: def add_symmetric(self, issuer, key, usage=None):
          
          if issuer not in self.issuer_keys:
              self.issuer_keys[issuer] = []
  
          if usage is None:
              self.issuer_keys[issuer].append(
                  self.keybundle_cls([{"kty": "oct", "key": key}]))
          else:
              for use in usage:
                  self.issuer_keys[issuer].append(
                      self.keybundle_cls([{"kty": "oct",
                                           "key": key,
                                           "use": use}]))
docs: Add a symmetric key. This is done by wrapping it in a key bundle 
          cloak since KeyJar does not handle keys directly but only through
          key bundles.
          
          :param issuer: Owner of the key
          :param key: The key 
          :param usage: What the key can be used for signing/signature 
              verification (sig) and/or encryption/decryption (enc)