$\{\}$ $\{a,b,c\}$ $\{a\}$ $\{b\}$ $\{c\}$ $\{a,c\}$ $\{a,b\}$ $\{b,c\}$ $write(\{a\})$: $read()$: $\sqsupseteq \{a\} \to$ OK $\sqsupseteq \{a\} \to$ OK $\sqsupseteq \{a\}$ $\to \{\}$ return OK return $\{\}\sqcup\{a\}=\{a\}$ $\to \{a\}$