can a citadel user ecplain to me what public pool is ? TY nostr:nevent1qqsyf3pec9e7naf3zjzxuz2ywkjz5cyve7x05spfdxa2uzhkwqxzdfqpzpmhxue69uhkummnw3ezuamfdejsyg9e55m4ywa69lx6aptajrv2wcx7fusnnj0epkvx7aruulgwcrgh85psgqqqqqqslqfvje