$\{\}$ $\{a,b,c\}$ $\{a\}$ $\{b\}$ $\{c\}$ $\{a,c\}$ $\{a,b\}$ $\{b,c\}$ $write(\{a\})$: $\not\sqsupseteq \{a\}$ $\not\sqsupseteq \{a\}$ $\not\sqsupseteq \{a\}$