code: func (s *SigCache) Exists(sigHash chainhash.Hash, sig *btcec.Signature, pubKey *btcec.PublicKey) bool {
  	s.RLock()
  	entry, ok := s.validSigs[sigHash]
  	s.RUnlock()
  
  	return ok && entry.pubKey.IsEqual(pubKey) && entry.sig.IsEqual(sig)
  }
docs: // Exists returns true if an existing entry of 'sig' over 'sigHash' for public
  // key 'pubKey' is found within the SigCache. Otherwise, false is returned.
  //
  // NOTE: This function is safe for concurrent access. Readers won't be blocked
  // unless there exists a writer, adding an entry to the SigCache.