code: func anyClientRedisErrSpec(spec *MethodSpec, method string) *MethodSpec {
  	// Commands that can return ERR given random input and state
  	switch method {
  	// Redis-Spec: Background save may be running already and can raise -ERR
  	case "Bgsave":
  		spec.NoRedisErr = false
  	}
  
  	return spec
  }
docs: // spec.NoRedisErr
  // Specs MethodSpec.NoRedisErr - client type invariant